Specify in 2.16 release notes that /p/ isn't supported anymore

'/p/' was introduced back in Gerrit 2.1.2 as a way to use the SmartHTTP
protocol instead of the dumb one.

From ReleaseNotes/ReleaseNotes-2.1.2.txt

"Enable smart HTTP under /p/ URLs

Git 1.6.6 and later support a more efficient HTTP protocol for both
fetch/clone and push, by relying upon Git specific server-side logic.
Gerrit Code Review now includes the necessary server-side support when
accessing repositories using URLs of the form
`http://review.example.com/p/'$projectname'.git`.
Authentication over smart HTTP URLs is performed using standard HTTP
digest authentication, with the username matching the SSH username."

Because the digest authentication isn't supported anymore, any legacy
application using the /p/ prefix wouldn't work anyway.

Current applications and clients should use the /a/ prefix instead
and switch to basic authentication.

P.S. The /p/ prefix has also been taken by the PolyGerrit U.I. and
goes partially in conflict with the legacy syntax.

Bug: Issue 10381
Change-Id: I6a86d8b39875f401ac3509b4400e65664579407a
diff --git a/pages/site/releases/2.16.md b/pages/site/releases/2.16.md
index 42cc1d0..d6e02ce 100644
--- a/pages/site/releases/2.16.md
+++ b/pages/site/releases/2.16.md
@@ -145,6 +145,11 @@
 [receive.allowPushToRefsChanges](https://gerrit-documentation.storage.googleapis.com/Documentation/2.16/config-gerrit.html#receive.allowPushToRefsChanges)
 to true.
 
+### Legacy `/p/` prefix for Git/HTTP projects is removed
+
+The `/p/` cannot be used anymore and the `/a/` prefix should be used for Git/HTTP
+with basic authentication.
+
 ### DELETE requests with bodies are no longer recommended
 
 Some proxies, which are not under client control, prohibit DELETE requests with