Documentation: kernel-doc: enumerate identifier *type*s

Explain that a kernel-doc :identifiers: line can refer to a struct,
union, enum, or typedef as well as functions.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20241119203201.110953-1-rdunlap@infradead.org