filectx: correct docstring about "changeid"

The changeid argument must be a revnum (basefile.rev() is defined as
"return self._changeid"), so fix the lie in the docstring. It seems to
have been incorrect for at least 10 years (I didn't check further
back).

Differential Revision: https://phab.mercurial-scm.org/D4881

mercurial/context.py

@@ -909,7 +909,7 b' class filectx(basefilectx):'
        filerevision convenient."""
     def __init__(self, repo, path, changeid=None, fileid=None,
                  filelog=None, changectx=None):
-        """changeid can be a changeset revision, node, or tag.
+        """changeid must be a revision number, if specified.
            fileid can be a file revision or node."""
         self._repo = repo
         self._path = path