Improve docs for search preferences (#32098)
Improve docs for search preferences (#32098)

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 and explains more clearly why
`_primary`, `_replica`, etc. are deprecated in `6.x` and removed in `master`.

Relates #31929 #26335 #26791.
strapdata/elassandra,vroyer/elassandra,strapdata/elassandra,vroyer/elassandra,strapdata/elassandra,vroyer/elassandra,strapdata/elassandra,strapdata/elassandra