helptext: document share safe functionality in `hg help config -v`

If share safe functionality is enabled, we read `.hg/hgrc' of shared source.

Differential Revision: https://phab.mercurial-scm.org/D9413

mercurial/helptext/config.txt

@@ -147,6 +147,15 b' be overridden in other configuration fil'
 merge tool configuration but packagers can also put other default configuration
 there.
 
+.. container:: verbose
+
+    On versions 5.7 and later, if share-safe functionality is enabled,
+    shares will read config file of share source too.
+    `<share-source/.hg/hgrc>` is read before reading `<repo/.hg/hgrc>`.
+
+    For configs which should not be shared, `<repo/.hg/hgrc-not-shared>`
+    should be used.
+
 Syntax
 ======