Customisable command synopsis in help message

I'd like to be able to customise the command synopsis a little bit more. The current synopsis generator in `UsageGenerator` will collapse all options into a single `` entry if there are more than twelve. My command has only 3 options, but with the long names being included in the synopsis, it's already unwieldy.

A few of ideas:

- Allow providing a manual synopsis.
- Similar to `.hidden`, allow hiding of options on a case-by-case basis.
- Collapse options with default values by default.

Start by reading the UsageGenerator synopsis generator and its current behavior when more than twelve options are present. Compare the requested approaches—manual synopsis, per-option hiding, and collapsing options with default values—and determine which behavior should be specified. Done means the chosen customization is implemented consistently and its synopsis output is covered by tests, though no test file is named in the issue.

Written by the indexing model from the issue text.