[MINOR][DOCS] Mention lack of RDD order preservation after deserialization
[MINOR][DOCS] Mention lack of RDD order preservation after deserialization

### What changes were proposed in this pull request?

This changes the docs to make it clearer that order preservation is not guaranteed when saving a RDD to disk and reading it back ([SPARK-5300](https://issues.apache.org/jira/browse/SPARK-5300)).

I added two sentences about this in the RDD Programming Guide.

The issue was discussed on the dev mailing list:
http://apache-spark-developers-list.1001551.n3.nabble.com/RDD-order-guarantees-td10142.html

### Why are the changes needed?

Because RDDs are order-aware collections, it is natural to expect that if I use `saveAsTextFile` and then load the resulting file with `sparkContext.textFile`, I obtain a RDD in the same order.

This is unfortunately not the case at the moment and there is no agreed upon way to fix this in Spark itself (see PR #4204 which attempted to fix this). Users should be aware of this.

### Does this PR introduce _any_ user-facing change?

Yes, two new sentences in the documentation.

### How was this patch tested?

By checking that the documentation looks good.

Closes #28465 from wetneb/SPARK-5300-docs.

Authored-by: Antonin Delpeuch <4c69575ae52ccaf13353fcd10ca26f77fb742b5e@delpeuch.eu>
Signed-off-by: Sean Owen <d519fa33a62684a61ab04b24d26f83452d51b34c@gmail.com>

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