Docs: Clarify/unify “stealing” docs

[encukou]

Documentation

For C functions that “steal” arguments:

• the behaviour on error should be explicitly mentioned for each function (see discussion);
• on top of that, the term “steal” deserves to be a glossary term that's linked from each use;
• the term always appears in quotes, see Poor terminology around reference ownership capi-workgroup/problems#11. (There's also a suggestion to avoid the term in prose entirely; that'll be out of scope for my PR.)

Linked PRs

• gh-144473: Add "steal" term to glossary; clarify "stealing" on error #144474