Update index-modules.asciidoc

Docs: Clarified `blocks.write` vs `blocks.read_only`

docs/reference/index-modules.asciidoc

@@ -180,7 +180,9 @@ specific index module:
 
 `index.blocks.write`::
 
-    Set to `true` to disable write operations against the index.
+    Set to `true` to disable data write operations against the index. Unlike `read_only', 
+    this setting does not affect metadata. For instance, you can close an index with a `write`
+    block, but not an index with a `read_only` block.
 
 `index.blocks.metadata`::