Some cleanup for sfxge.4

Use standard mdoc macros instead of pure roff, fix some other mdoc usage,
make the style consistent, and fix some grammar issues.

Approved by:	hrs (mentor)


git-svn-id: svn+ssh://svn.freebsd.org/base/head@272377 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f