better document macros taking literal strings

better document macros taking literal strings

When giving a function-style prototype for a macro taking a literal string
parameter, put a string literal in place of a type for that parameter.
This goofy appearance makes it obvious that this isn't really a function,
and clues the reader in that the parameter can't actually be an arbitrary
expression of the right type.  Also change the nonsensical "NUL-terminated
literal string" to "literal string" to describe these parameters.
Fixes [perl #116286].