histedit: add description about basic histedit function to command help

Before this patch, there is no explicit description that histedit
edits changesets between specified ancestor and the parent of the
working directory: users may notice it by error message "REV is not an
ancestor of working directory".

This patch adds description about basic histedit function to command
help of it.

This patch uses term "ancestor" instead of "parent", because it seems
to be more suitable, and almost all (error) messages already use it.

hgext/histedit.py

@@ -451,9 +451,12 b" actiontable = {'p': pick,"
      ('f', 'force', False,
       _('force outgoing even for unrelated repositories')),
      ('r', 'rev', [], _('first revision to be edited'))],
-     _("[PARENT]"))
+     _("ANCESTOR"))
 def histedit(ui, repo, *freeargs, **opts):
     """interactively edit changeset history
+
+    This command edits changesets between ANCESTOR and the parent of
+    the working directory.
     """
     # TODO only abort if we try and histedit mq patches, not just
     # blanket if mq patches are applied somewhere
@@ -493,7 +496,7 b' def histedit(ui, repo, *freeargs, **opts'
             revs.extend(freeargs)
             if len(revs) != 1:
                 raise util.Abort(
-                    _('histedit requires exactly one parent revision'))
+                    _('histedit requires exactly one ancestor revision'))
 
 
     if goal == 'continue':