Edits to WCAG 2.1 Introduction in place

Last week I submitted a pull request to update the Introduction to WCAG 
2.1. This reflects its anticipated status as a Proposed Recommendation, 
and also is meant to address concerns about coverage overclaims that 
were raised when we went to CR. Andrew accepted the pull request as an 
editorial change, but it's important for WG participants to be aware of 
this.

The new intro is available in the editors' draft:

http://w3c.github.io/wcag21/guidelines/

You can see the changes in the pull request, though it's a bit hard to read:

https://github.com/w3c/wcag21/pull/847/files

In summary:

  * There's a clause added to the abstract "...but do not address every
    user need" at the end of the sentence beginning "Following these
    guidelines will make content accessible to a wider range...".
  * I copied in the three sections from the WCAG 2.0 introduction that
    were previously only referenced with links, and which have
    information about the limitations of WCAG 2.0. I made minimal edits
    when copying to update them to 2.1 context.
  * The sections "New Features in WCAG 2.1", "Numbering in WCAG 2.1",
    and "Conformance to WCAG 2.1" are moved to subsections of a new
    section, "Comparison with WCAG 2.0".
  * The list of added success criteria now includes conformance levels.
  * There is a new section "User Support in WCAG 2.1" to describe what
    WCAG 2.1 does and does not do. Because we don't have the formal
    "Supplemental Guidance" yet, we can't reference that for additional
    information, so it references the WCAG Introduction at
    https://www.w3.org/WAI/intro/wcagwhich we would be able to keep up
    to date with references to additional guidance as it comes available.
  * The ednote about "we're working on clarifying guidance" which was
    added late to the CR abstract is removed, since these edits address
    that ednote.

If you have comments about the revised introduction, let me know. 
Although the changes are editorial, it's important positioning content 
that should have the support of the WG.

Michael