Improve help text on keybindings

[waf]

Improve help text to clarify the format for custom keybindings.

Fixes #363, #364

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.2%. Comparing base (de35670) to head (417227e).

Files with missing lines	Patch %	Lines
CSharpRepl.Services/Configuration.cs	0.0%	2 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #485   +/-   ##
=====================================
  Coverage   79.2%   79.2%           
=====================================
  Files        116     116           
  Lines       8596    8597    +1     
  Branches    1277    1277           
=====================================
+ Hits        6809    6810    +1     
  Misses      1350    1350           
  Partials     437     437           

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