Understanding docs to be revisisted for more complete Benefits sections

[mbgower]

While completing work on the In Brief sections for all WCAG criteria, @GreggVan, @rainbreaw, and others identified a number of Understanding documents where a user need is either absent or under developed.

The ones identified were as follows (linked to comments in the PR):

• Keyboard no exceptions
• Captions prerecorded
• audio-only live
• audio control
• audio description prerecorded
• consistent navigation
• low or no audio background
• media alternative prerecorded
• pause, stop, hide
• sensory characteristics
• change on request
• identify input purpose and input purpose

A few also gave more details on considerations for the Understanding document (beyond user benefit) that could be added, such as:

• language of parts
• Non-text content

These comments need to be reviewed and incorporated into the Understanding documents, where appropriate. The Benefits section tends to be the primary place where information could be added, but there may be other opportunities to address in some documents, or even new techniques to be considered.

[mbgower]

The discussion in https://github.com/w3c/wcag/issues/3385 may lead to additions to Reflow's Understanding document, so I have added it to this Issue for future Understanding document updates, as a result of the In brief work.

[mbgower]

Based on @gundulaniemann's comments in an AGWG survey and other's comments during the weekly call regarding assistive technology benefits in the two Input Purpose SCs, I have added those to the list.