Documentation: clearly specify what refs are honored by core.logAllRefUpdates

The documentation for logging updates in git-update-ref, doesn't make it
clear that only a specific subset of refs are honored by this variable.
Signed-off-by: NBert Wesarg <bert.wesarg@googlemail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -330,7 +330,9 @@ core.logAllRefUpdates::
 	SHA1, the date/time and the reason of the update, but
 	only when the file exists.  If this configuration
 	variable is set to true, missing "$GIT_DIR/logs/<ref>"
-	file is automatically created for branch heads.
+	file is automatically created for branch heads (i.e. under
+	refs/heads/), remote refs (i.e. under refs/remotes/),
+	note refs (i.e. under refs/notes/), and the symbolic ref HEAD.
 +
 This information can be used to determine what commit
 was the tip of a branch "2 days ago".

Documentation/git-update-ref.txt

@@ -60,8 +60,9 @@ still contains <oldvalue>.
 
 Logging Updates
 ---------------
-If config parameter "core.logAllRefUpdates" is true or the file
-"$GIT_DIR/logs/<ref>" exists then `git update-ref` will append
+If config parameter "core.logAllRefUpdates" is true and the ref is one under
+"refs/heads/", "refs/remotes/", "refs/notes/", or the symbolic ref HEAD; or
+the file "$GIT_DIR/logs/<ref>" exists then `git update-ref` will append
 a line to the log file "$GIT_DIR/logs/<ref>" (dereferencing all
 symbolic refs before creating the log name) describing the change
 in ref value.  Log lines are formatted as: