doc: add note about resource type in async_hooks

Hi everyone 👋

During the last Diagnostic WG meeting, the question has been raised of the type argument interface (async_hooks / init hook).

The initial demand was to document more precisely each type. But @bmeck mentions that this list of identifiers described in the documentation could arbitrarily change.

So, this pull request contains a note to explicitly tell users what is the status of these identifiers.

CC @RafaelGSS / @gireeshpunathil