Clarify preference docs
This commit clarifies the preference docs regarding the explanation of
how operations are routed by default. In particular, the previous use of
"shard replicas" was confusing as it could imply an operation would only
be routed to replicas by default.
Relates
[[search-request-preference]]
=== Preference
-Controls a `preference` of which shard replicas to execute the search
-request on. By default, the operation is randomized between the shard
-replicas.
+Controls a `preference` of which shard copies on which to execute the
+search. By default, the operation is randomized among the available shard copies.
The `preference` is a query string parameter which can be set to:
