gdb: Extend the comments in c-exp.y

gdb: Extend the comments in c-exp.y

In an attempt to fix PR gdb/13368 this commit adds some comments to
c-exp.y which hopefully makes the type parsing code a little clearer.
There are no code changes here, so there should be no user visible
changes after this commit.

gdb/ChangeLog:

	PR gdb/13368
	* c-exp.y (typebase): Extend the comment.
	(ident_tokens): Likewise.