docs(ngShowHide): make a note of values considered to be falsy

This issue has been a focus of problems for some users and we discussed it on the IRC that it should
be at least documented.

~Amended the style to use bootstrap notes, I think overall it looks better and catches the eyes more
easily. However there are no anchor links to these, if these are necessary they can be added later.

Closes #3436
Closes #5762