help: give hint about 'hg help -e' when appropriate

The hint is only given if a command shadows an extension with the same
name and when that extension has a multi-line module docstring.

mercurial/commands.py

@@ -2223,6 +2223,18 b' def help_(ui, name=None, with_version=Fa'
 
             addglobalopts(False)
 
+        # check if this command shadows a non-trivial (multi-line)
+        # extension help text
+        try:
+            mod = extensions.find(name)
+            doc = gettext(mod.__doc__) or ''
+            if '\n' in doc.strip():
+                msg = _('use "hg help -e %s" to show help for '
+                        'the %s extension') % (name, name)
+                ui.write('\n%s\n' % msg)
+        except KeyError:
+            pass
+
     def helplist(header, select=None):
         h = {}
         cmds = {}

tests/test-extension.t

@@ -254,6 +254,8 b' Extension module help vs command help:'
       compared to the working directory, and, when no revisions are specified,
       the working directory files are compared to its parent.
   
+  use "hg help -e extdiff" to show help for the extdiff extension
+  
   options:
   
    -p --program CMD          comparison program to run