commands: more informative description of "!" files in status

This makes the description more similar to the one used by Subversion.
Suggestion by Dennis Brakhane.

mercurial/commands.py

@@ -2748,7 +2748,7 b' def status(ui, repo, *pats, **opts):'
     A = added
     R = removed
     C = clean
-    ! = missing, but still tracked
+    ! = missing (deleted by non-hg command, but still tracked)
     ? = not tracked
     I = ignored
       = the previous added file was copied from here

tests/test-help.out

@@ -269,7 +269,7 b' show changed files in the working direct'
     A = added
     R = removed
     C = clean
-    ! = missing, but still tracked
+    ! = missing (deleted by non-hg command, but still tracked)
     ? = not tracked
     I = ignored
       = the previous added file was copied from here