Indices PUT Mapping API docs: Remove mapping type user and rephrase first sentence (#28998)
Indices PUT Mapping API docs: Remove mapping type user and rephrase first sentence (#28998)

The current docs on [Indices APIs: PUT Mapping](https://www.elastic.co/guide/en/elasticsearch/reference/current/indices-put-mapping.html) suggests that a having number of different mapping types per index is still possible in elasticsearch versions > 6.0.0 although they have been [removed](https://www.elastic.co/guide/en/elasticsearch/reference/current/removal-of-types.html). The console code has already been updated accordingly but notes (2) and (3) on the console code still name the `user` mapping type.

This PR updates the list with notes after the console code, as well as the first sentence of the docs
to avoid confusion. Also, I have removed the second command from the console code as it no
longer holds any value if the docs are solely on the `_doc` mapping.
gfyoung/elasticsearch,HonzaKral/elasticsearch,GlenRSmith/elasticsearch,gingerwizard/elasticsearch,GlenRSmith/elasticsearch,robin13/elasticsearch,coding0011/elasticsearch,GlenRSmith/elasticsearch,rajanm/elasticsearch,uschindler/elasticsearch,rajanm/elasticsearch,gingerwizard/elasticsearch,HonzaKral/elasticsearch,rajanm/elasticsearch,nknize/elasticsearch,nknize/elasticsearch,kalimatas/elasticsearch,uschindler/elasticsearch,gfyoung/elasticsearch,gingerwizard/elasticsearch,nknize/elasticsearch,kalimatas/elasticsearch,s1monw/elasticsearch,coding0011/elasticsearch,gfyoung/elasticsearch,rajanm/elasticsearch,s1monw/elasticsearch,gfyoung/elasticsearch,coding0011/elasticsearch,uschindler/elasticsearch,rajanm/elasticsearch,nknize/elasticsearch,kalimatas/elasticsearch,robin13/elasticsearch,scorpionvicky/elasticsearch,coding0011/elasticsearch,gfyoung/elasticsearch,scorpionvicky/elasticsearch,rajanm/elasticsearch,kalimatas/elasticsearch,gingerwizard/elasticsearch,HonzaKral/elasticsearch,robin13/elasticsearch,gingerwizard/elasticsearch,s1monw/elasticsearch,scorpionvicky/elasticsearch,HonzaKral/elasticsearch,GlenRSmith/elasticsearch,uschindler/elasticsearch,coding0011/elasticsearch,gingerwizard/elasticsearch,GlenRSmith/elasticsearch,s1monw/elasticsearch,gingerwizard/elasticsearch,robin13/elasticsearch,robin13/elasticsearch,uschindler/elasticsearch,s1monw/elasticsearch,scorpionvicky/elasticsearch,scorpionvicky/elasticsearch,nknize/elasticsearch,kalimatas/elasticsearch