Clarify working of enums and typedefs to them in LinkDefs

[hahnjo]

Follow-up from #13238 (comment), see also the three points raised in the linked comment.

From a user perspective, a #pragma link C++ enum currently produces neither an error nor a warning if the enum is not found (this is going to change with LLVM 16). As far as I can tell, typedefs to unnamed enums are currently not considered (this was the pattern addressed in #13238), it is not clear if they should.

[hahnjo]

according to https://root.cern/manual/io_custom_classes/#selecting-dictionary-entries-linkdefh:

In the past, linkdef files also contained directives for global variables, functions and enums: these directives are ignored since ROOT version 6.