bcache: clarify free/available/unused space

Don't describe bcache_available_percent as free space but as
non-writeback space.  Describe priority_stats in more detail
and point to that for total bcache occupation.

Signed-off-by: Gabriel de Perthuis <g2p.code+bcache@gmail.com>
Signed-off-by: Kent Overstreet <koverstreet@google.com>