CLEANUP: http: add a few comments on certain functions' assumptions about streams

CLEANUP: http: add a few comments on certain functions' assumptions about streams

get_http_auth() expects a valid stream but this is not mentioned, though
fortunately it's always called from places which already check this.

smp_prefetch_htx() performs all the required checks and is the key to the
stability of almost all sample fetch functions, so let's make this clearer.