Now that we've made accessor macro expansion more lazy, ensure that
when querying for init accessors (e.g., to build a memberwise
initializer), we also expand any accessor macros that might produce an
init accessor.
This is a partial step toward the real goal, which is that
`AbstractStorageDecl::getAccessor()` should lazily expand macros if
needed.
Update the Observable macro to document that it produces an `init`
accessor.