[WebLayer] Describe constraint of not exposing Content API publicly

Simple addition to //weblayer/public/README.md.

Change-Id: I7c01fbfdcb78a12202d34462c6ec2220468f89ae
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1916459
Commit-Queue: Colin Blundell <blundell@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715295}

weblayer/public/README.md

 # WebLayer public API
 
 This directory contains the public API for WebLayer. WebLayer provides both a
-C++ and Java API.
+C++ and Java API. Note that while WebLayer's implementation builds on top of
+//content, its public API does *not* expose the Content API.
 
 ## Java API