Edits to Content on Hover or Focus

[steverep]

Changes for this pull request can be viewed at: http://rawgit.com/w3c/wcag21/hover-focus-edits/guidelines/#content-on-hover-or-focus

The following changes are made:

1. Revision to Hoverable condition to use more plain language (issues The Paciello Group comment on 1.4.13 Content on Hover or Focus #695 and Intopia Feedback – SC 1.4.13 Content on Hover or Focus #711)
2. Revised initial sentence to scope to additional content which also disappears on hover or focus. This is needed to scope out content that may appear on focus such as in a tab list, drop down box, etc. (issue Success Criterion 1.4.13 Content on Hover or Focus #650)
3. Adds an exception to the Dismissable condition when the additional content doesn't obscure or replace anything. This addresses comments on mailing list regarding things like help content inserted inline when an input field has focus.

[joshueoconnor]

@steverep All looks good to me :-)