Add an explanation for the flags used by FS_IOC_[GS]ETFLAGS and remind
people that changes should be revised by linux-fsdevel and linux-api.

Add flags that are used on-disk for ext4, and remove FS_DIRECTIO_FL
since it was used only by gfs2 and support was removed in 2008 in
commit c9f6a6bb ("The ability to mark files for direct i/o access
when opened normally is both unused and pointless, so this patch
removes support for that feature.")  Now we have _two_ remaining flags
left.  But since we want to discourage people from assigning new
flags, that's OK.
Signed-off-by: NTheodore Ts'o <tytso@mit.edu>