Documentation: enhance branch.<name>.{remote,merge}

The documentation for branch.*.merge is very dense, so add a simple
explanation on top of it.

And branch.*.remote also affects 'git push'.

Signed-off-by: Santi Béjar <santi@agolina.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -473,10 +473,14 @@ branch.autosetuprebase::
 	This option defaults to never.
 
 branch.<name>.remote::
-	When in branch <name>, it tells 'git-fetch' which remote to fetch.
-	If this option is not given, 'git-fetch' defaults to remote "origin".
+	When in branch <name>, it tells 'git-fetch' and 'git-push' which
+	remote to fetch from/push to.  It defaults to `origin` if no remote is
+	configured. `origin` is also used if you are not on any branch.
 
 branch.<name>.merge::
+	Defines, together with branch.<name>.remote, the upstream branch
+	for the given branch. It tells 'git-fetch'/'git-pull' which
+	branch to merge from.
 	When in branch <name>, it tells 'git-fetch' the default
 	refspec to be marked for merging in FETCH_HEAD. The value is
 	handled like the remote part of a refspec, and must match a