DOC: htx: make it clear that htxbuf() and htx_from_buf() always return valid pointers

DOC: htx: make it clear that htxbuf() and htx_from_buf() always return valid pointers

Update the comments above htxbuf() and htx_from_buf() to make it clear
that they always return valid htx pointers so that callers know they do
not have to test them. This is only true after the fix on b_is_null()
which was the only known corner case.

This should be backported to 1.9 if the b_is_null() patch is backported.