Document custom profile fields API

[gnprice]

The "custom profile fields" feature is an area where our API docs aren't up to our usual standard; a number of items within it are wrong, and some things that are necessary for using the feature aren't described, so that the implementer has to resort to reverse-engineering.

This isn't an urgent priority to fix; I already did that reverse-engineering last year and wrote down the results in the zulip-mobile codebase. Filing this so there's an issue number to refer to when mentioning this gap.

For details on what's wrong and what's missing, see

• the chat thread #api design > custom profile fields, particularly the strings of messages I sent starting here and here; and
• the related chat thread #api design > external account custom profile fields.

[zulipbot]

Hello @zulip/server-api, @zulip/server-settings members, this issue was labeled with the "area: settings (user)", "area: documentation (api and integrations)" labels, so you may want to check it out!