btrfs: add comments for btrfs_reserve_flush_enum

This enum is the interface exposed to developers.

Although we have a detailed comment explaining the whole idea of space
flushing at the beginning of space-info.c, the exposed enum interface
doesn't have any comment.

Some corner cases, like BTRFS_RESERVE_FLUSH_ALL and
BTRFS_RESERVE_FLUSH_ALL_STEAL can be interrupted by fatal signals, are
not explained at all.

So add some simple comments for these enums as a quick reference.

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