[SPARK-28790][DOC][SQL] Document CACHE TABLE statement in SQL Reference
[SPARK-28790][DOC][SQL] Document CACHE TABLE statement in SQL Reference

### What changes were proposed in this pull request?
Document CACHE TABLE statement in SQL Reference

### Why are the changes needed?
To complete SQL Reference

### Does this PR introduce any user-facing change?
Yes.

Here is the screen shot:

![image](https://user-images.githubusercontent.com/13592258/64072307-26f45c80-cc41-11e9-8ab3-dc56fe8ff45f.png)

![image](https://user-images.githubusercontent.com/13592258/64072309-2cea3d80-cc41-11e9-9a4d-8cb9eb63569f.png)

### How was this patch tested?
Tested using jykyll build --serve

Closes #25532 from huaxingao/spark-28790.

Authored-by: Huaxin Gao <b1011540267ee7fe5a02243ad09c007ca1860809@us.ibm.com>
Signed-off-by: Xiao Li <905db6c57c9ba2de5402239d136139da9b5d0058@gmail.com>

cloud-fan/spark,goldmedal/spark,caneGuy/spark,bdrillard/spark,ptkool/spark,rednaxelafx/apache-spark,wangmiao1981/spark,witgo/spark,caneGuy/spark,dongjoon-hyun/spark,hvanhovell/spark,JoshRosen/spark,dongjoon-hyun/spark,vinodkc/spark,zzcclp/spark,dbtsai/spark,xuanyuanking/spark,dbtsai/spark,taroplus/spark,kevinyu98/spark,srowen/spark,skonto/spark,ConeyLiu/spark,wangyum/spark,witgo/spark,dbtsai/spark,WeichenXu123/spark,cloud-fan/spark,srowen/spark,jkbradley/spark,HyukjinKwon/spark,dbtsai/spark,pgandhi999/spark,jkbradley/spark,vinodkc/spark,zero323/spark,bdrillard/spark,WeichenXu123/spark,shuangshuangwang/spark,ptkool/spark,WeichenXu123/spark,milliman/spark,skonto/spark,xuanyuanking/spark,dbtsai/spark,zero323/spark,witgo/spark,shaneknapp/spark,chuckchen/spark,pgandhi999/spark,HyukjinKwon/spark,dbtsai/spark,dongjoon-hyun/spark,bdrillard/spark,wzhfy/spark,rednaxelafx/apache-spark,taroplus/spark,bdrillard/spark,dongjoon-hyun/spark,WeichenXu123/spark,vinodkc/spark,ConeyLiu/spark,holdenk/spark,shaneknapp/spark,mahak/spark,vinodkc/spark,jiangxb1987/spark,darionyaphet/spark,cloud-fan/spark,gengliangwang/spark,wzhfy/spark,ptkool/spark,pgandhi999/spark,zuotingbing/spark,zero323/spark,srowen/spark,gengliangwang/spark,xuanyuanking/spark,ptkool/spark,mahak/spark,JoshRosen/spark,rezasafi/spark,goldmedal/spark,ueshin/apache-spark,kevinyu98/spark,caneGuy/spark,kevinyu98/spark,witgo/spark,jkbradley/spark,JoshRosen/spark,zzcclp/spark,holdenk/spark,maropu/spark,rednaxelafx/apache-spark,BryanCutler/spark,apache/spark,shaneknapp/spark,zuotingbing/spark,wzhfy/spark,ConeyLiu/spark,milliman/spark,JoshRosen/spark,shaneknapp/spark,skonto/spark,HyukjinKwon/spark,xuanyuanking/spark,holdenk/spark,caneGuy/spark,spark-test/spark,wangyum/spark,darionyaphet/spark,ueshin/apache-spark,milliman/spark,maropu/spark,vinodkc/spark,maropu/spark,taroplus/spark,ptkool/spark,caneGuy/spark,mahak/spark,srowen/spark,wangmiao1981/spark,vinodkc/spark,kevinyu98/spark,BryanCutler/spark,ConeyLiu/spark,ueshin/apache-spark,goldmedal/spark,wangyum/spark,witgo/spark,rezasafi/spark,skonto/spark,darionyaphet/spark,mahak/spark,jiangxb1987/spark,cloud-fan/spark,skonto/spark,WeichenXu123/spark,goldmedal/spark,zero323/spark,nchammas/spark,BryanCutler/spark,spark-test/spark,wangmiao1981/spark,jiangxb1987/spark,zero323/spark,mahak/spark,darionyaphet/spark,nchammas/spark,rezasafi/spark,BryanCutler/spark,xuanyuanking/spark,jkbradley/spark,gengliangwang/spark,pgandhi999/spark,shuangshuangwang/spark,bdrillard/spark,shuangshuangwang/spark,rednaxelafx/apache-spark,jiangxb1987/spark,zuotingbing/spark,goldmedal/spark,darionyaphet/spark,ptkool/spark,apache/spark,nchammas/spark,wangmiao1981/spark,spark-test/spark,maropu/spark,apache/spark,wangmiao1981/spark,shaneknapp/spark,cloud-fan/spark,caneGuy/spark,hvanhovell/spark,jkbradley/spark,rednaxelafx/apache-spark,darionyaphet/spark,goldmedal/spark,skonto/spark,bdrillard/spark,cloud-fan/spark,zzcclp/spark,ConeyLiu/spark,ConeyLiu/spark,gengliangwang/spark,zzcclp/spark,zzcclp/spark,jkbradley/spark,hvanhovell/spark,gengliangwang/spark,darionyaphet/spark,gengliangwang/spark,chuckchen/spark,xuanyuanking/spark,JoshRosen/spark,rednaxelafx/apache-spark,taroplus/spark,wzhfy/spark,holdenk/spark,holdenk/spark,nchammas/spark,milliman/spark,taroplus/spark,maropu/spark,kevinyu98/spark,hvanhovell/spark,WeichenXu123/spark,shaneknapp/spark,wangyum/spark,BryanCutler/spark,ueshin/apache-spark,pgandhi999/spark,taroplus/spark,nchammas/spark,ueshin/apache-spark,jkbradley/spark,zero323/spark,dbtsai/spark,HyukjinKwon/spark,jiangxb1987/spark,chuckchen/spark,holdenk/spark,ConeyLiu/spark,spark-test/spark,jiangxb1987/spark,wzhfy/spark,caneGuy/spark,wzhfy/spark,dongjoon-hyun/spark,kevinyu98/spark,apache/spark,srowen/spark,wangyum/spark,maropu/spark,chuckchen/spark,xuanyuanking/spark,taroplus/spark,HyukjinKwon/spark,HyukjinKwon/spark,zuotingbing/spark,ptkool/spark,WeichenXu123/spark,ueshin/apache-spark,wzhfy/spark,mahak/spark,shuangshuangwang/spark,spark-test/spark,holdenk/spark,matthewfranglen/spark,spark-test/spark,milliman/spark,zzcclp/spark,rezasafi/spark,JoshRosen/spark,JoshRosen/spark,pgandhi999/spark,milliman/spark,pgandhi999/spark,maropu/spark,apache/spark,kevinyu98/spark,shaneknapp/spark,zuotingbing/spark,hvanhovell/spark,zero323/spark,cloud-fan/spark,wangyum/spark,gengliangwang/spark,apache/spark,rezasafi/spark,goldmedal/spark,rednaxelafx/apache-spark,dongjoon-hyun/spark,witgo/spark,jiangxb1987/spark,milliman/spark,HyukjinKwon/spark,chuckchen/spark,hvanhovell/spark,bdrillard/spark,rezasafi/spark,chuckchen/spark,srowen/spark,dongjoon-hyun/spark,ueshin/apache-spark,wangmiao1981/spark,skonto/spark,mahak/spark,BryanCutler/spark,nchammas/spark,zuotingbing/spark,shuangshuangwang/spark,hvanhovell/spark,rezasafi/spark,witgo/spark,spark-test/spark,zuotingbing/spark,shuangshuangwang/spark,wangmiao1981/spark,zzcclp/spark,wangyum/spark,vinodkc/spark,shuangshuangwang/spark,BryanCutler/spark,nchammas/spark,chuckchen/spark,srowen/spark,apache/spark