3.2.6 Consistent Help Understanding: clarify "change initiated by the user"

[dbjorge]

Rendered version (see mostly-new "Limitations and Exceptions" section)

This PR is intended to address #3226 by adding some additional clarification and examples based on WCAG 2 backlog meeting discussion + these two comments [1] [2].

Summary of the new explanations:

• Different "sets of web pages" are allowed to have different help mechanism locations (but it's best to be as consistent as possible)
• The SC text excepting cases where "a change is initiated by a user" is intended to cover cases where a user performs an action with the intent of changing the display or layout of a page, such as changing the zoom level, orientation, or viewport size
• That exception is not intended to treat treat every action that a user might take as a "user-initiated change" - specifically, navigating between pages within a set of web pages doesn't count, and neither does logging into a page (unless doing so would present the user with a new set of web pages)

The main "intent" section was getting pretty long with these additions, so I created a new "Limitations and Exceptions" subsection. I moved a few other existing paragraphs that felt relevant to the new section.

[scottaohara]

this update covers exactly what i was hoping to have clarified. thank you.

[alastc]

Approved: https://www.w3.org/2023/09/19-ag-minutes#t06