[imorland]

Refs flarum/framework#5236.

Changes proposed in this pull request:
Explain that secondary tags stay alphabetical and cannot be reordered within their list. Document that changing an existing tag’s isPrimary returns 422, while sending its current value remains valid; changing kind uses the tag-order endpoint.

Reviewers should focus on:

• The API guidance for changing a tag between primary and secondary.

Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered?
• For core PRs, does this need to be in core, or could it be in an extension?
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Frontend changes: tests are green (run yarn test in js/).
• Frontend changes: tests have been added, or are not appropriate here.
• Backend changes: tests are green (run composer test).
• Backend changes: tests have been added, or are not appropriate here.
• Where applicable, changes are suitable for all supported database drivers (MySQL, MariaDB, PostgreSQL, SQLite).
• Core developer confirmed locally this works as intended.
• The description above is written by me and describes what this pull request actually does.