docs: do not promise a per-call permission prompt we have not verified

The page said the browser asks before any agent calls a tool. Prompt
granularity is browser-specific and unsettled during the origin trial, and
we have not observed it on the native path. Say what holds, that access is
gated, and name the part that is still moving.

Author: miguel-heygen

docs/guides/webmcp.mdx

@@ -138,8 +138,9 @@ replaces the whole preferences blob and loses your panel sizes, zoom and timelin
 
 Set it back to `true`, or delete the key, to re-enable.
 
-The browser also asks your permission before any agent actually calls a tool, so registering them is
-not the same as granting access.
+The browser gates tool access behind its own permission prompt, so registering a tool is not the same
+as granting access to it. How often you are asked, once per site or every call, is up to the browser
+and is still changing while the API is in origin trial.
 
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