Spelling, grammar and readability fixups

Spelling, grammar and readability in upgrader documents
  - Tweaked spelling, grammar and wording to make the text of the upgrader
    pages more readable.
Fixed spelling regression
  - Higly —> Highly
Spelling and readability fixes for /include/class.* files
  - Fixed some spelling mistakes
A couple more spelling/readability fixes.
  - More fixes
Merge pull request #617 from greezybacon/issue/grammar

Spelling, grammar and readability fixups

Reviewed-With: Jared Hancock <jared@osticket.com>