Belatedly add documentation for crossbow flag

[chaosvolt]

Summary

SUMMARY: Content "Belatedly remember to add CROSSBOW flag to JSON_FLAGS documentation"

Purpose of change

Tiny thing I forgot with #1526.

Describe the solution

Added the crossbow flag to the documentation.

Describe alternatives you've considered

1. Forgetting about it for another five years.
2. More serious answer, Coolthuhlu has mentioned that the whole "gunmods are filtered by both gun type and slot type" is a bit clunky and might be worth overhauling, but for now should at least document how it works currently. An overhaul is gonna be a lot more work.

Testing

...how does one even test documentation other than reading it, anyway.

Additional context