help/config: expand [subpaths] help

Brifly explain why rewriting subrepository paths can be necessary.
Explain that relative subrepository paths are made absolute before
rewrite rules are applied.

mercurial/help/config.txt

@@ -1052,20 +1052,24 b' Configuration for extensions that need t'
 ``subpaths``
 """"""""""""
 
-Defines subrepositories source locations rewriting rules of the form::
+Subrepository source URLs can go stale if a remote server changes name
+or becomes temporarily unavailable. This section lets you define
+rewrite rules of the form::
 
     <pattern> = <replacement>
 
-Where ``pattern`` is a regular expression matching the source and
-``replacement`` is the replacement string used to rewrite it. Groups
-can be matched in ``pattern`` and referenced in ``replacements``. For
-instance::
+where ``pattern`` is a regular expression matching a subrepository
+source URL and ``replacement`` is the replacement string used to
+rewrite it. Groups can be matched in ``pattern`` and referenced in
+``replacements``. For instance::
 
     http://server/(.*)-hg/ = http://hg.server/\1/
 
 rewrites ``http://server/foo-hg/`` into ``http://hg.server/foo/``.
 
-All patterns are applied in definition order.
+Relative subrepository paths are first made absolute, and the the
+rewrite rules are then applied on the full (absolute) path. The rules
+are applied in definition order.
 
 ``trusted``
 """""""""""