geli.8: minor proofreading

Sponsored by: Axcient
Reviewed by: imp, pauamma (manpages)
Differential Revision: https://reviews.freebsd.org/D44907

(cherry picked from commit c0f02dcd4c9c17181c59fa3c43259cce793a0b21)

geli.8: add some notes regarding performance tuning

Sponsored by: Axcient
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D44908

(cherry picked from commit 4b7949144cfbf25b0e0bf593b135a88417304d71)