credential-store: document the file format a bit more

Reading a malformed credential URL line and silently ignoring it
does not mean that we support empty lines and/or "# commented" lines
forever.  We should document it to avoid confusion.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-credential-store.txt

@@ -94,6 +94,10 @@ stored on its own line as a URL like:
 https://user:pass@example.com
 ------------------------------
 
+No other kinds of lines (e.g. empty lines or comment lines) are
+allowed in the file, even though some may be silently ignored. Do
+not view or edit the file with editors.
+
 When Git needs authentication for a particular URL context,
 credential-store will consider that context a pattern to match against
 each entry in the credentials file.  If the protocol, hostname, and