[SPARK-15502][DOC][ML][PYSPARK] add guide note that ALS only supports integer ids
[SPARK-15502][DOC][ML][PYSPARK] add guide note that ALS only supports integer ids

This PR adds a note to clarify that the ML API for ALS only supports integers for user/item ids, and that other types for these columns can be used but the ids must fall within integer range.

(Refer [SPARK-14891](https://issues.apache.org/jira/browse/SPARK-14891)).

Also cleaned up a reference to `mllib` in the ML doc.

## How was this patch tested?
Built and viewed User Guide doc locally.

Author: Nick Pentreath <nickp@za.ibm.com>

Closes #13278 from MLnick/SPARK-15502-als-int-id-doc-note.

jlopezmalla/spark,JoshRosen/spark,techaddict/spark,zuotingbing/spark,gengliangwang/spark,nlalevee/spark,caneGuy/spark,JerryLead/spark,dbtsai/spark,kissmetrics/spark,holdenk/spark,ptkool/spark,mdespriee/spark,UndeadBaneGitHub/spark,nchammas/spark,szhem/spark,jianran/spark,ptkool/spark,kevinyu98/spark,bdrillard/spark,debugger87/spark,akopich/spark,shuangshuangwang/spark,shuangshuangwang/spark,panfengfeng/spark,ahnqirage/spark,apache-spark-on-k8s/spark,mzl9039/spark,aokolnychyi/spark,actuaryzhang/spark,jianran/spark,vinodkc/spark,kiszk/spark,stanzhai/spark,ahnqirage/spark,saturday-shi/spark,caneGuy/spark,joseph-torres/spark,shaneknapp/spark,dbtsai/spark,ueshin/apache-spark,maropu/spark,panfengfeng/spark,bdrillard/spark,HyukjinKwon/spark,michalsenkyr/spark,1haodian/spark,adrian-ionescu/apache-spark,guoxiaolongzte/spark,wangyum/spark,techaddict/spark,janewangfb/spark,wangmiao1981/spark,bdrillard/spark,milliman/spark,rezasafi/spark,rezasafi/spark,alunarbeach/spark,cloud-fan/spark,shubhamchopra/spark,vinodkc/spark,hvanhovell/spark,sachintyagi22/spark,SHASHANKB/spark,mdespriee/spark,janewangfb/spark,minixalpha/spark,publicRoman/spark,darionyaphet/spark,loneknightpy/spark,shaneknapp/spark,huang1900/spark,srowen/spark,brad-kaiser/spark,akopich/spark,wangmiao1981/spark,esi-mineset/spark,holdenk/spark,SHASHANKB/spark,rezasafi/spark,saltstar/spark,big-pegasus/spark,milliman/spark,aokolnychyi/spark,cin/spark,chuckchen/spark,aosagie/spark,shuangshuangwang/spark,witgo/spark,stanzhai/spark,wangyum/spark,andrewor14/spark,brad-kaiser/spark,Panos-Bletsos/spark-cost-model-optimizer,poffuomo/spark,bOOm-X/spark,loneknightpy/spark,pgandhi999/spark,JerryLead/spark,huang1900/spark,shubhamchopra/spark,dbtsai/spark,ajaysaini725/spark,big-pegasus/spark,stanzhai/spark,ericvandenbergfb/spark,icexelloss/spark,poffuomo/spark,kimoonkim/spark,byakuinss/spark,highfei2011/spark,Aegeaner/spark,zhouyejoe/spark,eyalfa/spark,ibm-research-ireland/sparkoscope,hvanhovell/spark,LantaoJin/spark,ibm-research-ireland/sparkoscope,tejasapatil/spark,chuckchen/spark,tejasapatil/spark,HyukjinKwon/spark,taroplus/spark,rednaxelafx/apache-spark,setjet/spark,stanzhai/spark,aray/spark,panfengfeng/spark,JoshRosen/spark,someorz/spark,akopich/spark,kiszk/spark,rekhajoshm/spark,jiangxb1987/spark,witgo/spark,mike0sv/spark,ajaysaini725/spark,vinodkc/spark,brad-kaiser/spark,metamx/spark,ptkool/spark,bravo-zhang/spark,ahnqirage/spark,eyalfa/spark,michalsenkyr/spark,aokolnychyi/spark,saltstar/spark,milliman/spark,highfei2011/spark,janewangfb/spark,mahak/spark,BryanCutler/spark,jlopezmalla/spark,rekhajoshm/spark,facaiy/spark,narahari92/spark,Panos-Bletsos/spark-cost-model-optimizer,Aegeaner/spark,kiszk/spark,UndeadBaneGitHub/spark,dhruve/spark,yanboliang/spark,narahari92/spark,lxsmnv/spark,aray/spark,bdrillard/spark,setjet/spark,gengliangwang/spark,nchammas/spark,facaiy/spark,minixalpha/spark,actuaryzhang/spark,holdenk/spark,jrshust/spark,bOOm-X/spark,taroplus/spark,darionyaphet/spark,janewangfb/spark,map222/spark,actuaryzhang/spark,zero323/spark,caneGuy/spark,rednaxelafx/apache-spark,jianran/spark,zhouyejoe/spark,tejasapatil/spark,rednaxelafx/apache-spark,witgo/spark,jkbradley/spark,ibm-research-ireland/sparkoscope,minixalpha/spark,mike0sv/spark,yanboliang/spark,lxsmnv/spark,wangmiao1981/spark,ron8hu/spark,szhem/spark,rezasafi/spark,apache/spark,metamx/spark,guoxiaolongzte/spark,dongjoon-hyun/spark,kevinyu98/spark,xflin/spark,SnappyDataInc/spark,ueshin/apache-spark,esi-mineset/spark,eyalfa/spark,jlopezmalla/spark,sahilTakiar/spark,wangmiao1981/spark,guoxiaolongzte/spark,Aegeaner/spark,ConeyLiu/spark,markhamstra/spark,bOOm-X/spark,zuotingbing/spark,aray/spark,bdrillard/spark,zhouyejoe/spark,actuaryzhang/spark,yanboliang/spark,Panos-Bletsos/spark-cost-model-optimizer,lxsmnv/spark,WeichenXu123/spark,sachintyagi22/spark,kevinyu98/spark,holdenk/spark,JoshRosen/spark,sahilTakiar/spark,Panos-Bletsos/spark-cost-model-optimizer,shuangshuangwang/spark,mahak/spark,zero323/spark,BryanCutler/spark,someorz/spark,kimoonkim/spark,dhruve/spark,ron8hu/spark,jrshust/spark,dbtsai/spark,lxsmnv/spark,pgandhi999/spark,loneknightpy/spark,milliman/spark,byakuinss/spark,maropu/spark,ahnqirage/spark,liutang123/spark,JoshRosen/spark,mdespriee/spark,WeichenXu123/spark,darionyaphet/spark,shubhamchopra/spark,JerryLead/spark,wzhfy/spark,wzhfy/spark,cin/spark,SHASHANKB/spark,zzcclp/spark,skonto/spark,loneknightpy/spark,chuckchen/spark,caneGuy/spark,apache-spark-on-k8s/spark,markhamstra/spark,adrian-ionescu/apache-spark,rikima/spark,saturday-shi/spark,debugger87/spark,pgandhi999/spark,patrick-nicholson/spark,darionyaphet/spark,ConeyLiu/spark,big-pegasus/spark,patrick-nicholson/spark,icexelloss/spark,mzl9039/spark,guoxiaolongzte/spark,icexelloss/spark,apache-spark-on-k8s/spark,MLnick/spark,dongjoon-hyun/spark,ioana-delaney/spark,Aegeaner/spark,mdespriee/spark,caneGuy/spark,ericvandenbergfb/spark,debugger87/spark,tengpeng/spark,ericvandenbergfb/spark,ajaysaini725/spark,zuotingbing/spark,publicRoman/spark,liu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