Add comments and rename intro_nodes list in rend_services_introduce()

(No changes file needed: this patch just adds comments and renames
variables. This is ticket 13646. message taken from the ticket. -Nick)
Check descriptor ID in addition to HS ID when saving a v2 hs descriptor

Fixes bug 13214; reported by 'special'.
Merge remote-tracking branch 'public/bug13214_025_squashed'