os_support: fix misleading comments

cherry-picked from speexdsp 86779a06f6500d041573d6252d4971d3bfcb4b18
Ogg Opus updates from Ian Nartowicz.

These are the changes proposed in
 <http://www.ietf.org/mail-archive/web/codec/current/msg03058.html>,
 with some minor additions (see follow-up on the list).