revset: document wdir() as an experimental function

Let's resurrect the docstring since our help module can detect the EXPERIMENTAL
tag and display it only if -v is specified.

This patch updates the test added by 015c0d1087a3 since wdir() is now
documented.

mercurial/revset.py

@@ -2259,9 +2259,9 b' def user(repo, subset, x):'
     """
     return author(repo, subset, x)
 
-# experimental
 @predicate('wdir', safe=True)
 def wdir(repo, subset, x):
+    """Working directory. (EXPERIMENTAL)"""
     # i18n: "wdir" is a keyword
     getargs(x, 0, 0, _("wdir takes no arguments"))
     if node.wdirrev in subset or isinstance(subset, fullreposet):

tests/test-revset.t

@@ -2809,8 +2809,8 b' Bogus function with a similar internal n'
   [255]
 
 Undocumented functions aren't suggested as similar either
-  $ log 'wdir2()'
-  hg: parse error: unknown identifier: wdir2
+  $ log 'tagged2()'
+  hg: parse error: unknown identifier: tagged2
   [255]
 
 multiple revspecs