Update docs to match implementation that resource names are rooted at the package. Ref #1635.

changelog.d/1635.change.rst

+Resource paths are passed to ``pkg_resources.resource_string`` and similar no longer accept paths that traverse parents. Violations of this expectation raise DeprecationWarnings and will become errors.

docs/pkg_resources.txt

@@ -1132,8 +1132,8 @@ relative to the root of the identified distribution; i.e. its first path
 segment will be treated as a peer of the top-level modules or packages in the
 distribution.
 
-Note that resource names must be ``/``-separated paths and cannot be absolute
-(i.e. no leading ``/``) or contain relative names like ``".."``.  Do *not* use
+Note that resource names must be ``/``-separated paths rooted at the package
+and cannot contain relative names like ``".."``.  Do *not* use
 ``os.path`` routines to manipulate resource paths, as they are *not* filesystem
 paths.
 

pkg_resources/__init__.py

@@ -1489,8 +1489,7 @@ class NullProvider:
         >>> warned.clear()
         >>> vrp('/foo/bar.txt')
         >>> bool(warned)
-        True
-        >>> warned.clear()
+        False
         >>> vrp('foo/../../bar.txt')
         >>> bool(warned)
         True
@@ -1499,14 +1498,11 @@ class NullProvider:
         >>> bool(warned)
         False
         """
-        invalid = (
-            path.startswith('/') or
-            re.search(r'\B\.\.\B', path)
-        )
+        invalid = '..' in path.split('/')
         if not invalid:
             return
 
-        msg = "Use of .. or leading / in a resource path is not allowed."
+        msg = "Use of .. in a resource path is not allowed."
         # for compatibility, warn; in future
         # raise ValueError(msg)
         warnings.warn(