Document feature for generating images with all SCAs

[bmorris3]

On main, users can generate images for all SCAs using romanisim-make-image ... --sca -1 ..., in a case that gets caught here:

romanisim/scripts/romanisim-make-image

Lines 27 to 37 in e0e6a97

	if args.sca == -1:
	# simulate all 18 SCAs sequentially
	origfilename = copy(args.filename)
	origprevious = copy(args.previous)
	for i in range(1, 19):
	args.sca = i
	args.filename = origfilename.format(f'WFI{args.sca:02d}')
	if origprevious is not None:
	args.previous = origprevious.format(f'WFI{args.sca:02d}')
	go(args)
	return

This feature isn't written about in the docs or in the argument's help notes, so I've added a note to both.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.44%. Comparing base (0a1aac5) to head (1bf6bd7).
Report is 62 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #129      +/-   ##
==========================================
- Coverage   90.78%   87.44%   -3.34%     
==========================================
  Files          17       17              
  Lines        1671     1689      +18     
==========================================
- Hits         1517     1477      -40     
- Misses        154      212      +58     

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[schlafly]

Thank you!