Security guide editorial edits

No textual changes, just markup and style, mainly:
* Add missing <filename> markup
* Change capitalization
* Wrap some overly long lines
* Remove extra whitespace before ":"
* Replace "--" with emdash

Change-Id: I9126a91c5d1f78feacb10c639b26f37fc6815a99
Partial-Bug: #1217503
Merge "Security guide editorial edits"