userguide.adoc: edit for grammar, spelling and phrasing
userguide.adoc: simplify step 1 instructions (copying files to Saved Games\DCS)
userguide.adoc: remove Prolog section

The section referred to additional documentation that does not yet exist,
so I removed it for now.
userguide.adoc: edit for technical accuracy
Merge pull request #28 from jboecker/userguide-edits

User Guide edits