Update userguide.adoc
Update userguide.adoc

Text changes made but need basic proof reading for typo's.  Picture to be added next time.
userguide.adoc: small corrections
User Guide: add A-10C-specific notes

Refs #19
Refs #14
Merge pull request #22 from jboecker/userguide-updates

Userguide updates: Rewrites, clarifications, A-10C notes