libctf: dedup: describe 'citer'

libctf: dedup: describe 'citer'

The distinction between the citer and citers variables in
ctf_dedup_rhash_type is somewhat opaque (it's a micro-optimization to avoid
having to allocate entire sets when we know in advance that we'll only have
to store one value).  Add a comment.

libctf/
	* ctf-dedup.c (ctf_dedup_rhash_type): Comment on citers variables.