[DOCS] Unclear that frozen indices are read-only #50092
Description
As noted in #46110, the Freeze index API docs don't explicitly use the term read-only. However, this is implied in the following sentence:
A frozen index has almost no overhead on the cluster (except for maintaining its metadata in memory), and is blocked for write operations.
Explicitly using read-only would make it clear that frozen indices are eligible for read-only index operations, like forge merges.