Improve comments in print-utils.h.

Improve comments in print-utils.h.

Since I had to look at these function comments to fix the RISC-V ARI warnings,
I noticed that they make no sense.  The pulongest and plongest comments are
swapped.  phex is missing a comment.  And phex_nz doesn't mention how it is
different from phex.

	* gdbsupport/print-utils.h (pulongest): Fix comment.
	(plongest): Likewise.
	(phex): Add missing comment, mention leading zeros.
	(phex_nz): Add mention of no leading zeros to comment.