libctf: archive: comment improvements

libctf: archive: comment improvements

A file format-defining header such as ctf.h should not describe
internal implementation details of how some fields are reused inside libctf.

Describe the modent table as what it is, an array, and mention that it is
sorted by name.

include/
	* ctf.h (struct ctf_archive) [ctfa_modents]: Mention sort order.
	[ctfa_magic]: Improve comment.
	(struct ctf_archive_v1) [ctfa_magic]: Likewise.  Mention the
	off-the-end nature of the modent array in this format.