fix: when `show_default` is a string use it as the default description

[muggenhor]

Because this value doesn't just function as a boolean. When it's a string it's intended to allow the user to override the documentation of the default.

This parameter's alternative value is documented in https://click.palletsprojects.com/en/7.x/api/?highlight=show_default#click.Option

[codecov-io]

Codecov Report

Merging #50 into master will decrease coverage by 17.2%. The diff coverage is 66.66%.

@@             Coverage Diff             @@
##           master      #50       +/-   ##
===========================================
- Coverage    71.9%   54.69%   -17.21%     
===========================================
  Files           1        1               
  Lines         210      298       +88     
  Branches       57      105       +48     
===========================================
+ Hits          151      163       +12     
- Misses         50      128       +78     
+ Partials        9        7        -2

Impacted Files	Coverage Δ
sphinx_click/ext.py	54.69% <66.66%> (-17.21%)	:arrow_down:

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[stephenfin]

Thanks. This has been released as part of 2.3.0.

[muggenhor]

Thanks for merging this so quickly :-)