options: Add contextual value to the helper text

[amyspark]

👋

This is a PR to add a placeholder in the options' display text so that the user knows what to enter as values.

[codecov]

Codecov Report

Attention: 21 lines in your changes are missing coverage. Please review.

Comparison is base (29962af) 89.57% compared to head (1fba992) 89.18%.

Files	Patch %	Lines
impl/command_line/options.cxx	51.16%	21 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #82      +/-   ##
==========================================
- Coverage   89.57%   89.18%   -0.39%     
==========================================
  Files          47       47              
  Lines        3424     3459      +35     
  Branches      662      669       +7     
==========================================
+ Hits         3067     3085      +18     
- Misses        321      338      +17     
  Partials       36       36              

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[dragonmux]

This is looking pretty great! There are a couple of stylistic items we've spotted in review and a couple of queries about lambda captures, but with those addressed we're happy to get this merged.

[amyspark]

I've applied all the bits as requested. Let me know if anything else is needed!

[dragonmux]

LGTM, we'll get this merged once the builds complete. Thank you for the contribution!