docs(api): load a relationship in full with withoutRelatedScope()

[imorland]

Fixes: no issue.

Changes proposed in this pull request:
From 2.0.2, a relationship field can call withoutRelatedScope() so that endpoints eager load it in full rather than through the related resource's scope (flarum/framework#5241). The API page gains a "Loading a relationship in full" section under Relationships: when to use it (a relation that policies read, such as a user's groups), and that the field's getter must then leave out what the actor may not see.

Reviewers should focus on:

• This should merge with, or after, fix(core): count hidden groups when checking a serialised user's permissions framework#5241.

Screenshot
N/A, text only.

Necessity

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

Confirmed

• Frontend changes: tested on a local Flarum installation. (No code changes.)
• Frontend changes: tests are green (run yarn test in js/). (No code changes.)
• Frontend changes: tests have been added, or are not appropriate here. (No code changes.)
• Backend changes: tests are green (run composer test). (No code changes.)
• Backend changes: tests have been added, or are not appropriate here. (No code changes.)
• Where applicable, changes are suitable for all supported database drivers (MySQL, MariaDB, PostgreSQL, SQLite). (N/A)
• Core developer confirmed locally this works as intended.
• The description above is written by me and describes what this pull request actually does.