Change "check-box" to "checkbox" in the docs.
Review Request #5539 — Created Feb. 24, 2014 and submitted
The existing docs were using "check-box," which is wrong. I guess I
added this ages ago, but I'm not sure why.This replaces all occurrences with "checkbox."
Built the docs and saw the changes.
Grepped through all the docs for all occurrences.
