libctf: add a comment explaining how to use ctf_*open

Specifically, tell users what to pass to those functions that accept raw
section content, since it's fairly involved and easy to get wrong.
(.dynsym / .dynstr when CTF_F_DYNSTR is set, otherwise .symtab / .strtab).

include/ChangeLog:

	* ctf-api.h (ctf_*open): Improve comment.