X.L.Spacing + X.L.Gaps: add mutual hyperlink

These layout have different applications but their names could cause some
confusion since the module names could just as well be swapped. To support this claim,
what we call "spacing" is named "gaps" in i3wm (i3-gaps). Therefore hyperlinks have
been added to inform the reader of the existence of the other module.

XMonad/Layout/Gaps.hs

@@ -14,7 +14,9 @@
 -- be used for tiling, along with support for toggling gaps on and
 -- off.
 --
--- Note that "XMonad.Hooks.ManageDocks" is the preferred solution for
+-- Note 1: For gaps\/space around /windows/ see "XMonad.Layout.Spacing".
+--
+-- Note 2: "XMonad.Hooks.ManageDocks" is the preferred solution for
 -- leaving space for your dock-type applications (status bars,
 -- toolbars, docks, etc.), since it automatically sets up appropriate
 -- gaps, allows them to be toggled, etc.  However, this module may

XMonad/Layout/Spacing.hs

@@ -12,6 +12,8 @@
 -- Portability :  portable
 --
 -- Add a configurable amount of space around windows.
+--
+-- Note: For space\/gaps along edges of the /screen/ see "XMonad.Layout.Gaps".
 -----------------------------------------------------------------------------
 
 module XMonad.Layout.Spacing (