Amends #2392: help text improvements

[chucklorenz]

This PR changes _htmlClassName to _htmlClasses. It clarifies some Titles in various schemas and adds some Help texts.

Reference #2392

[moloko]

Totally agree this is much clearer, my only comment is that because _htmlClassName has been available in a released version of the FW (v4.3.0) you're introducing a breaking change by doing this.

You could either make this change only v5 adapt - or add a fallback and include it in both.

Because it's only been available for a short while I think the actual risk of it breaking anything is very low, so for simplicity's sake I'd slightly favour changing this as part of v5...

[chucklorenz]

Updated this PR to return to use of _htmlClassName so that the amends to Titles and Help could continue toward master. I'll create another PR for _htmlClasses against release/v5. (PR has been created: #2555)

[Link2Twenty]

One thing to mention, developers used to JSX and React will already be used to classes being called className. If we go down the path of using React for the front end it may not be necessary to change the property name.