[DOCS] Clarify frozen indices are read-only (elastic#50318)

The freeze index API docs state that frozen indices are blocked for
write operations.

While this implies frozen indices are read-only, it does not explicitly
use the term "read-only", which is found in other docs, such as the
force merge docs.

This adds the "ready-only" term to the freeze index API docs as well
as other clarification.

Author: jrodewig

docs/reference/indices/apis/freeze.asciidoc

@@ -19,9 +19,10 @@ Freezes an index.
 [[freeze-index-api-desc]]
 ==== {api-description-title}
 
-A frozen index has almost no overhead on the cluster (except
-for maintaining its metadata in memory), and is blocked for write operations.
-See <<frozen-indices>> and <<unfreeze-index-api>>.
+A frozen index has almost no overhead on the cluster (except for maintaining its
+metadata in memory) and is read-only. Read-only indices are blocked for write
+operations, such as <<indexing,docs-index_>> or <<indices-forcemerge,force
+merges>>. See <<frozen-indices>> and <<unfreeze-index-api>>.
 
 IMPORTANT: Freezing an index will close the index and reopen it within the same
 API call. This causes primaries to not be allocated for a short amount of time