upgrade command-line docs for sourmash sig subcommands

[ctb]

The various sourmash sig subcommands (cat, extract, etc) do not give very useful information when invoked on the command line without any arguments --

% sourmash sig cat
usage:  cat [-h] [-q] [-o FILE] [-u] signatures [signatures ...]
 cat: error: the following arguments are required: signatures

to address this:

• pick a sourmash sig subcommand that doesn't have useful output when run w/o arguments
• find the relevant subcommand file (e.g. src/sourmash/cli/sig/cat.py)
• in that file, add a description=__doc__ and usage=usage argument to the subparser.add_parser(...) call; see src/sourmash/compute.py for an example
• initialize the __doc__ and usage strings from the command-line docs
• ask for initial review and someone will work with you on the text

This can also be done for many other commands than just the sourmash sig, too ;).

[ctb]

relevant: #899

[keyabarve]

@ctb Can I work on this issue?

[ctb]

yep!