Expand documentation of common-utils.h::FUNCTION_NAME

Expand documentation of common-utils.h::FUNCTION_NAME

While looking at this macro, I noticed that it wasn't always necessarily
defined. That prompted me to search the current sources to make sure
that all uses were adequately protected, which they were. But to help
prevent future uses to be made unprotected, this patch expands the
current macro documentation a bit.

gdb/ChangeLog:

        * common/common-utils.h (FUNCTION_NAME): Expand the macro's
        documentation a bit.