btrfs: add comments for btrfs_map_block()

The function btrfs_map_block() is a critical part of the btrfs storage
layer, which handles mapping of logical ranges to physical ranges.

Thus it's better to have some basic explanation, especially on the
following points:

- Segment split by various boundaries
  As a continuous logical range may be split into different segments,
  due to various factors like zones and RAID0/5/6/10 boundaries.

- The meaning of @mirror_num

- The possible single stripe optimization

- One deprecated parameter @need_raid_map
  Just explicitly mark it deprecated so we're aware of the problem.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>