mkfs: manpage fixups in config file section

mkfs: manpage fixups in config file section

A bit more clarification in the CONFIGURATION FILE FORMAT
section of the mkfs.xfs manpage.

Primarily, map the commandline switches to the section names
explicitly.

"optional default" sounds pretty weird, so call it the
"defaults" configuration file instead.

Document the comment format by example.

Fix the sparse inode default setting in the example.

Change the .BI tags to .B (there is no bold/italic switching).

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>