af: readme tweaks for web support [SDK-4217]

[stevehobbsdev]

• [ ] All new/changed/fixed functionality is covered by tests (or N/A)
• [ ] I have added documentation for all new/changed functionality (or N/A)

📋 Changes

PR includes a number of readme and documentation tweaks for the forthcoming web support:

• includes API ref for web
• rewords common tasks
• includes web-related details for credentials and hasCredentials

[Widcket]

Should the base of this PR be beta? Currently it's main.

[stevehobbsdev]

Woops! Fixed :)