adjust documentation for signature outputs to talk about memory usage and streaming

[ctb]

In #1493, we added support for saving signatures to more than just .sig or .sig.gz files. A slightly hidden consequence of this is that

(a) zipfile and directory saving can happen progressively with e.g. prefetch, so that signatures will be output "as you go" and not held in memory.
(b) that's not true of the .sig and .sig.gz formats, which require that the signatures be held in memory.

This should be mentioned somewhere in the docs!

[ctb]

added to command-line.md in #2184 commit fb45b6c:

Note that when outputting large collections of signatures, some save
formats require holding all the sketches in memory until they can be
written out, and others can save progressively. This can affect memory
usage! Currently .sig and .sig.gz formats are held in memory,
while .zip, directory outputs, and .sqldb formats write progressively
to disk.