Docs change for exists query. (#43092)
Docs change for exists query. (#43092)

Now emphasises the test is for indexed values.
Previous documentation only mentioned the state of the input JSON doc (null values) but this is only one of several reasons why an indexed value may not exist.

Closes #24256
gingerwizard/elasticsearch,gingerwizard/elasticsearch,robin13/elasticsearch,GlenRSmith/elasticsearch,coding0011/elasticsearch,HonzaKral/elasticsearch,coding0011/elasticsearch,gingerwizard/elasticsearch,robin13/elasticsearch,gingerwizard/elasticsearch,robin13/elasticsearch,gingerwizard/elasticsearch,uschindler/elasticsearch,nknize/elasticsearch,nknize/elasticsearch,GlenRSmith/elasticsearch,nknize/elasticsearch,HonzaKral/elasticsearch,uschindler/elasticsearch,nknize/elasticsearch,robin13/elasticsearch,scorpionvicky/elasticsearch,scorpionvicky/elasticsearch,gingerwizard/elasticsearch,coding0011/elasticsearch,HonzaKral/elasticsearch,uschindler/elasticsearch,uschindler/elasticsearch,scorpionvicky/elasticsearch,coding0011/elasticsearch,scorpionvicky/elasticsearch,gingerwizard/elasticsearch,coding0011/elasticsearch,HonzaKral/elasticsearch,robin13/elasticsearch,uschindler/elasticsearch,GlenRSmith/elasticsearch,GlenRSmith/elasticsearch,scorpionvicky/elasticsearch,GlenRSmith/elasticsearch,nknize/elasticsearch