Improve dandi ls --help and dandi download --help output

[jwodder]

Closes #823.

[codecov]

Codecov Report

Merging #827 (e4ff4c8) into master (d42938c) will decrease coverage by 0.28%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #827      +/-   ##
==========================================
- Coverage   85.48%   85.20%   -0.29%     
==========================================
  Files          61       61              
  Lines        6298     6298              
==========================================
- Hits         5384     5366      -18     
- Misses        914      932      +18     

Flag	Coverage Δ
unittests	85.20% <100.00%> (-0.29%)	⬇️

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Impacted Files	Coverage Δ
dandi/cli/cmd_download.py	92.30% <100.00%> (ø)
dandi/cli/cmd_ls.py	69.71% <100.00%> (ø)
dandi/dandiarchive.py	86.34% <100.00%> (-1.47%)	⬇️
dandi/support/generatorify.py	0.00% <0.00%> (-16.10%)	⬇️
dandi/dandiapi.py	89.00% <0.00%> (-0.18%)	⬇️

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[yarikoptic]

Looks and works great, thank you @jwodder !