xfs: document the XFS_ALLOC_AGFL_RESERVE constant

Source kernel commit: 66c8a2411c707c0b34319a26e556e461b2f360d4

Currently, we use this undocumented macro to encode the minimum number
of blocks needed to replenish a completely empty AGFL when an AG is
nearly full.  This has lead to confusion on the part of the maintainers,
so let's document what the value actually means, and move it to
xfs_alloc.c since it's not used outside of that module.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>