Update documentation for <webview> site isolation.

Various places in our documentation currently reference the lack of
site isolation in <webview> tags.  Update all such places now that
site isolation for <webview> is available.

Bug: 1267977
Change-Id: I50094307c63fc621b96e5833274035f2a9694739
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4385895
Reviewed-by: Nasko Oskov <[email protected]>
Commit-Queue: Alex Moshchuk <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1126059}
diff --git a/docs/webui_explainer.md b/docs/webui_explainer.md
index 4a5c3e2..3147bec 100644
--- a/docs/webui_explainer.md
+++ b/docs/webui_explainer.md
@@ -1181,8 +1181,7 @@
 
 Alternatively, a `<webview>` tag can be used, which runs in a separate
 StoragePartition, a separate frame tree, and restricts postMessage communication
-by default. However, `<webview>` does not support Site Isolation and
-therefore it is not advisable to use for any sensitive content.
+by default. Note that `<webview>` is only available on desktop platforms.
 
 ## JavaScript Error Reporting