Add warning that StreamParser prefix and comment events are not synchronized to data

[RubenVerborgh]

#423 introduced the comment event on StreamParser. In #439, I realized that comment emitted by StreamParser is not guaranteed to be synchronized. That is, there is no guarantee that an event timeline of "data" "comment" "data" means that the comment in question was actually positioned in between the two quads in the source file.

Similarly, the prefix event is also not reliable.

This is inherently unfixable with ReadStream<Quad> (only fixable with ReadStream<Quad|Prefix|Comment>), so we should document this. Consumers MUST NOT rely on synchronization, because there is none.

This is less harmful for usage of prefix and comment for non-location-specific information purposes (as would nearly always be the case for prefixes). If any prefix or comment handling pertains to a specific position, StreamParser usage must be replaced by Parser, which does provide order guarantees.