Document the self.alt pseudo-host for self-requests

[itowlson]

This will ship in 3.2 - do not merge until then.

Content must go through a pre-merge checklist.

Pre-Merge Content Checklist

This documentation has been checked to ensure that:

• The title, template, and date are all set
• Does this PR have a new menu item (anywhere in templates/*.hbs files) that points to a document .md that is set to publish in the future? If so please only publish the .md and .hbs changes in real-time (otherwise there will be a menu item pointing to a .md file that does not exist)
• File does not use CRLF, but uses plain LF (hint: use cat -ve <filename> | grep $'\r' | wc -l and expect 0 as a result)
• Has passed bart check
• Has been manually tested by running in Spin/Bartholomew (hint: use PREVIEW_MODE=1 and run npm run styles to update styling)
• Headings are using Title Case
• Code blocks have the programming language set to properly highlight syntax and the proper copy directive
• Have tested with npm run test and resolved all errors
• Relates to an existing (potentially outdated) blog article? If so please add URL in blog to point to this content.

[itowlson]

Ref spinframework/spin-js-sdk#298