🎨 Improve help message (#18, #19)

[edornd]

• add required or default info to fields
• add subcommand metavar for cleaner help

[codecov-commenter]

Codecov Report

Merging #20 (924414c) into main (eb8f671) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main       #20   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           15        15           
  Lines          473       485   +12     
=========================================
+ Hits           473       485   +12     

Impacted Files	Coverage Δ
argdantic/convert.py	100.00% <100.00%> (ø)
argdantic/core.py	100.00% <100.00%> (ø)
argdantic/utils.py	100.00% <100.00%> (ø)

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