Add a setter for FormHelper::context()

Having a setter makes changing context mid form possible. This is
sometimes useful when forms are partially filled with data from plugins.

Refs #4149
Update TimeHelper docs.

Refs #4133
Fix Time API docs.

Refs #4133
Remove incorrect reference to strftime.
Fix doc block error.
Merge pull request #4160 from markstory/issue-4149

3.0 add context setter and improve time helper docs.