Document `fromstream` best practices

fromstream's behavior is generally confusing, see the below issues:

• #1957
• #2464
• #3037

It seems like this filter requires end-of-object markers. How well-supported is the [[null]] trick? It would be nice if it were documented somewhere.

Alternatively, it would be nice to have a more intuitive filter that collects a stream into a single object. This way, it doesn't require end-of-object markers, so it never produces empty results.

def collectstream(f):
    fromstream(
        (f | select(length == 2)),
        [[null]]
    );