Hygiene: Document why selectors are needed

This rule has a lot of selectors (note the fix me)
It's not clear what each of them are needed for so make that
clearer in the code.

Change-Id: I80f07118977f465b7d09efad28f94917dd962a3d

resources/skins.minerva.base.styles/ui.less

@@ -313,10 +313,15 @@ input.search {
 
 @media all and ( min-width: @width-breakpoint-tablet ) {
 	// FIXME: this should be one generic class name
+	// SpecialMobileEditWatchlist
 	.page-summary-list,
+	// Talk overlay
 	.topic-title-list,
+	// structured languages, mobile pagelists
 	.site-link-list,
+	// overlays
 	.overlay .panel,
+	// language overlay / MobileSpecialPageFeed
 	.list-header {
 		padding-left: @contentPaddingTablet;
 		padding-right: @contentPaddingTablet;