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

Author: rddunlap

Documentation/doc-guide/kernel-doc.rst

@@ -533,6 +533,7 @@ identifiers: *[ function/type ...]*
   Include documentation for each *function* and *type* in *source*.
   If no *function* is specified, the documentation for all functions
   and types in the *source* will be included.
+  *type* can be a struct, union, enum, or typedef identifier.
 
   Examples::