[SPARK-30430][PYTHON][DOCS] Add a note that UserDefinedFunction's constructor is private
[SPARK-30430][PYTHON][DOCS] Add a note that UserDefinedFunction's constructor is private

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

This PR adds a note that UserDefinedFunction's constructor is private.

### Why are the changes needed?

To match with Scala side. Scala side does not have it at all.

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

Doc only changes but it declares UserDefinedFunction's constructor is private explicitly.

### How was this patch tested?

Jenkins

Closes #27101 from HyukjinKwon/SPARK-30430.

Authored-by: HyukjinKwon <2680965ad5aae29275c8fb10150cb10f5554e78b@apache.org>
Signed-off-by: HyukjinKwon <2680965ad5aae29275c8fb10150cb10f5554e78b@apache.org>

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