[Docs] Improve Convert Processor description (#35280)
[Docs] Improve Convert Processor description (#35280)

Sometimes users are confused about whether they can use the Convert Processor
for changing an existing fields type to other types even if the existing one is already
ingested. This confusion is from the first line of description. Changing this and also
adding a some detail to the code snippet.
gingerwizard/elasticsearch,GlenRSmith/elasticsearch,gingerwizard/elasticsearch,gingerwizard/elasticsearch,scorpionvicky/elasticsearch,HonzaKral/elasticsearch,robin13/elasticsearch,nknize/elasticsearch,coding0011/elasticsearch,robin13/elasticsearch,scorpionvicky/elasticsearch,HonzaKral/elasticsearch,robin13/elasticsearch,nknize/elasticsearch,nknize/elasticsearch,gingerwizard/elasticsearch,gingerwizard/elasticsearch,uschindler/elasticsearch,GlenRSmith/elasticsearch,uschindler/elasticsearch,scorpionvicky/elasticsearch,coding0011/elasticsearch,coding0011/elasticsearch,GlenRSmith/elasticsearch,nknize/elasticsearch,coding0011/elasticsearch,robin13/elasticsearch,scorpionvicky/elasticsearch,HonzaKral/elasticsearch,coding0011/elasticsearch,GlenRSmith/elasticsearch,gingerwizard/elasticsearch,uschindler/elasticsearch,uschindler/elasticsearch,GlenRSmith/elasticsearch,gingerwizard/elasticsearch,robin13/elasticsearch,nknize/elasticsearch,scorpionvicky/elasticsearch,uschindler/elasticsearch,HonzaKral/elasticsearch