config: document 'gui.GCWarning'

While investigating the config options set by 'scalar' I noticed this
one wasn't documented.

Signed-off-by: Matthew Hughes <matthewhughes934@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config/gui.adoc

diff --git a/Documentation/config/gui.adoc b/Documentation/config/gui.adoc
index 171be77..1565c0a 100644
--- a/Documentation/config/gui.adoc
+++ b/Documentation/config/gui.adoc
@@ -55,3 +55,8 @@
 	linkgit:gitk[1] for the selected commit, when the `Show History
 	Context` menu item is invoked from 'git gui blame'. If this
 	variable is set to zero, the whole history is shown.
+
+gui.GCWarning::
+	Determines whether linkgit:git-gui[1] should prompt for garbage
+	collection when git detects a large number of loose objects in
+	the repository. The default value is "true".