templatefilters: document the json filter

I don't know the exact reason why it was undocumented, but that would be
because unicode is difficult for humans. Since the json filter can now get
localstr back to UTF-8 as long as it is unprocessed (i.e. {desc|json} works,
but {desc|firstline|json} doesn't), it seems good enough to advertise the
json filter.

mercurial/templatefilters.py

@@ -238,6 +238,7 b' def indent(text, prefix):'
 
 @templatefilter('json')
 def json(obj, paranoid=True):
+    """Any object. Serializes the object to a JSON formatted text."""
     if obj is None:
         return 'null'
     elif obj is False: