Documentation/remote-helpers: document common use-case for private ref

The current documentation mentions the private ref namespace, but does
not really explain why it can be useful.

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/gitremote-helpers.txt

@@ -176,6 +176,12 @@ applicable refspec takes precedence.  The left-hand of refspecs
 advertised with this capability must cover all refs reported by
 the list command.  If no 'refspec' capability is advertised,
 there is an implied `refspec *:*`.
++
+When writing remote-helpers for decentralized version control
+systems, it is advised to keep a local copy of the repository to
+interact with, and to let the private namespace refs point to this
+local repository, while the refs/remotes namespace is used to track
+the remote repository.
 
 'bidi-import'::
 	This modifies the 'import' capability.