Improve docs for search preferences (#32159)
Improve docs for search preferences (#32159)

Today it is unclear what guarantees are offered by the search preference
feature, and we claim a guarantee that is stronger than what we really offer:

> A custom value will be used to guarantee that the same shards will be used
> for the same custom value.

This commit clarifies this documentation.

Forward-port of #32098 to `master`.
nknize/elasticsearch,uschindler/elasticsearch,scorpionvicky/elasticsearch,GlenRSmith/elasticsearch,coding0011/elasticsearch,HonzaKral/elasticsearch,gfyoung/elasticsearch,coding0011/elasticsearch,GlenRSmith/elasticsearch,GlenRSmith/elasticsearch,gfyoung/elasticsearch,gingerwizard/elasticsearch,uschindler/elasticsearch,GlenRSmith/elasticsearch,GlenRSmith/elasticsearch,gingerwizard/elasticsearch,gingerwizard/elasticsearch,scorpionvicky/elasticsearch,uschindler/elasticsearch,gingerwizard/elasticsearch,robin13/elasticsearch,gingerwizard/elasticsearch,robin13/elasticsearch,uschindler/elasticsearch,gfyoung/elasticsearch,HonzaKral/elasticsearch,uschindler/elasticsearch,robin13/elasticsearch,robin13/elasticsearch,gingerwizard/elasticsearch,nknize/elasticsearch,nknize/elasticsearch,gfyoung/elasticsearch,coding0011/elasticsearch,scorpionvicky/elasticsearch,gfyoung/elasticsearch,robin13/elasticsearch,scorpionvicky/elasticsearch,gingerwizard/elasticsearch,HonzaKral/elasticsearch,nknize/elasticsearch,scorpionvicky/elasticsearch,coding0011/elasticsearch,coding0011/elasticsearch,HonzaKral/elasticsearch,nknize/elasticsearch