[wv-docs] Document force dark default, and behaviour of FORCE_DARK_AUTO

This adds a reference to the View method that controls framework force
dark, which impacts how FORCE_DARK_AUTO is interpreted. It also
documents that FORCE_DARK_AUTO is the default.

Test: documentation only change
Fixes: 128704481
Change-Id: I88e7996362da5b092b269c313e37ef9253b12872