Bug 18541: (QA follow-up) Add some POD for draw_guide_grid

Add description to POD for draw_guide_grid

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Amended: Moved the description from draw_guide_box to .._grid.

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>

C4/Patroncards/Patroncard.pm

@@ -123,6 +123,10 @@ sub draw_guide_box {
 
 =head2 draw_guide_grid
 
+    $patron_card->draw_guide_grid($pdf)
+
+Adds a grid to the PDF output ($pdf) to support layout design
+
 =cut
 
 sub draw_guide_grid {