Add comments documenting ext2fs_[reserve_]super_and_bgd_loc()

Add comments documenting ext2fs_[reserve_]super_and_bgd_loc()

Add an explanation of exactly what ext2fs_super_and_bgd_loc() and
ext2fs_reserve_super_and_bgd_loc() do, and more importantly, exactly
what they return.  Note that most callers should *not* rely on the
return value since it's rarely useful, especially once the flex_bg
feature is enabled and inode table and allocation bitmap blocks may
not be in the block group.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>