repoview: improve documentation for `repo.filtered` method

repoview: improve documentation for `repo.filtered` method

I am sitting next to Joerg Sonnenberger and we are discussion his experience
with repoview. This first effect of this discussion is this documentation
clarification.

--- a/mercurial/localrepo.py	Fri Apr 05 14:30:52 2019 -0400
+++ b/mercurial/localrepo.py	Sat Apr 06 10:44:22 2019 +0200
@@ -1179,7 +1179,17 @@
         return self
 
     def filtered(self, name, visibilityexceptions=None):
-        """Return a filtered version of a repository"""
+        """Return a filtered version of a repository
+
+        The `name` parameter is the identifier of the requested view. This
+        will return a repoview object set "exactly" to the specified view.
+
+        This function does not apply recursive filtering to a repository. For
+        example calling `repo.filtered("served")` will return a repoview using
+        the "served" view, regardless of the initial view used by `repo`.
+
+        In other word, there is always only one level of `repoview` "filtering".
+        """
         cls = repoview.newtype(self.unfiltered().__class__)
         return cls(self, name, visibilityexceptions)