doc: document n-api callback scope usage
Document that it is not necessary to open handle and/or callback scopes inside finalizer, async work, thread-safe function etc. callbacks unless for reasons documented in the section about object lifetime management.
Link usage of callback signatures to their definition.
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
documentation is changed or added -
commit message follows commit guidelines