configitems: add `documentation` field

It may be useful to expose documentation information in the help in some form.

This will be populated in a future changeset by using the current comments
that are relevant for users.

mercurial/configitems.py

@@ -48,10 +48,12 b' class configitem:'
         generic=False,
         priority=0,
         experimental=False,
+        documentation="",
     ):
         self.section = section
         self.name = name
         self.default = default
+        self.documentation = documentation
         self.alias = list(alias)
         self.generic = generic
         self.priority = priority