help: document the `path://` url scheme

If we want people to use it, we need to document it.

Differential Revision: https://phab.mercurial-scm.org/D10372

mercurial/helptext/config.txt

@@ -1712,6 +1712,9 b' behavior for that specific path. Example'
     my_server = https://example.com/my_path
     my_server:pushurl = ssh://example.com/my_path
 
+Paths using the `path://otherpath` scheme will inherit the sub-options value from
+the path they point to.
+
 The following sub-options can be defined:
 
 ``pushurl``

mercurial/helptext/urls.txt

@@ -5,6 +5,7 b' Valid URLs are of the form::'
   http://[user[:pass]@]host[:port]/[path][#revision]
   https://[user[:pass]@]host[:port]/[path][#revision]
   ssh://[user@]host[:port]/[path][#revision]
+  path://pathname
 
 Paths in the local filesystem can either point to Mercurial
 repositories or to bundle files (as created by :hg:`bundle` or
@@ -64,3 +65,12 b' default:'
 default-push:
   The push command will look for a path named 'default-push', and
   prefer it over 'default' if both are defined.
+
+These alias can also be use in the `path://` scheme::
+
+  [paths]
+  alias1 = URL1
+  alias2 = path://alias1
+  ...
+
+check :hg:`help config.paths` for details about the behavior of such "sub-path".

tests/test-help.t

@@ -1842,6 +1842,9 b' Test section lookup'
         my_server = https://example.com/my_path
         my_server:pushurl = ssh://example.com/my_path
   
+      Paths using the 'path://otherpath' scheme will inherit the sub-options
+      value from the path they point to.
+  
       The following sub-options can be defined:
   
       "pushurl"