phases: add a formal note that hash of secret changeset may leak out

For technical reason (discovery, obsolescence marker) the hash of secret
changeset are communicated outside of your repo. We clarifie that in the help so
that people does not used hash of secret changeset as nuclear launch code.

mercurial/help/phases.txt

@@ -58,6 +58,13 b' See :hg:`help config` for more informati'
   Servers running older versions of Mercurial are treated as
   publishing.
 
+.. note::
+   Changesets in secret phase are not exchanged with the server. This
+   applies to their content: file names, file contents, and changeset
+   metadata. For technical reasons, the identifier (e.g. d825e4025e39)
+   of the secret changeset may be communicated to the server.
+
+
 Examples
 ========