X.A.Navigation2D: More prominently document strategies

Also, add a note that users who use gaps or spacing may need to look
into these strategies a bit more deeply.

Fixes: https://github.com/xmonad/xmonad-contrib/issues/627

XMonad/Actions/Navigation2D.hs

@@ -97,7 +97,14 @@ import XMonad.Util.Types
 -- >                              False
 -- >               $ def
 --
--- Alternatively, you can use navigation2DP:
+-- /NOTE/: the @def@ argument to 'navigation2D' contains the strategy
+-- that decides which windows actually get selected.  If you use modules
+-- that influence tiling in some way, like "XMonad.Layout.Spacing" or
+-- "XMonad.Layout.Gaps", you should think about using a different
+-- strategy, if you find the default behaviour to be unnatural.  Check
+-- out the [finer points](#g:Finer_Points) below for more information.
+--
+-- Alternatively to 'navigation2D', you can use 'navigation2DP':
 --
 -- > main = xmonad $ navigation2DP def
 -- >                               ("<Up>", "<Left>", "<Down>", "<Right>")
@@ -107,7 +114,7 @@ import XMonad.Util.Types
 -- >               $ def
 --
 -- That's it. If instead you'd like more control, you can combine
--- withNavigation2DConfig and additionalNav2DKeys or additionalNav2DKeysP:
+-- 'withNavigation2DConfig' and 'additionalNav2DKeys' or 'additionalNav2DKeysP':
 --
 -- > main = xmonad $ withNavigation2DConfig def
 -- >               $ additionalNav2DKeys (xK_Up, xK_Left, xK_Down, xK_Right)
@@ -178,9 +185,19 @@ import XMonad.Util.Types
 -- values in the above example to 'True'.  You could also decide you want
 -- wrapping only for a subset of the operations and no wrapping for others.
 --
--- By default, all layouts use the 'defaultTiledNavigation' strategy specified
+-- By default, all layouts use the 'defaultTiledNavigation' strategy
--- in the 'Navigation2DConfig' (by default, line navigation is used).  To
+-- specified in the 'Navigation2DConfig' (by default, line navigation is
--- override this behaviour for some layouts, add a pair (\"layout name\",
+-- used).  Many more navigation strategies are available; some may feel
+-- more natural, depending on the layout and user:
+--
+--   * 'lineNavigation'
+--   * 'centerNavigation'
+--   * 'sideNavigation'
+--   * 'sideNavigationWithBias'
+--
+-- There is also the ability to combine two strategies with 'hybridOf'.
+--
+-- To override the default behaviour for some layouts, add a pair (\"layout name\",
 -- navigation strategy) to the 'layoutNavigation' list in the
 -- 'Navigation2DConfig', where \"layout name\" is the string reported by the
 -- layout's description method (normally what is shown as the layout name in
@@ -326,7 +343,7 @@ centerNavigation = N 2 doCenterNavigation
 -- and push it to the right until it intersects with at least one other window.
 -- Of those windows, one with a point that is the closest to the centre of the
 -- line (+1) is selected. This is probably the most intuitive strategy for the
--- tiled layer when using XMonad.Layout.Spacing.
+-- tiled layer when using "XMonad.Layout.Spacing".
 sideNavigation :: Navigation2D
 sideNavigation = N 1 (doSideNavigationWithBias 1)