edk2_invocable.py: Improve documentation

[yangrongwei]

Improve the in-line documentation for edk2_invocable.py, which improves the quality of documentation at https://www.tianocore.org/edk2-pytool-extensions/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.76%. Comparing base (bf643b0) to head (e12d2b6).
Report is 38 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #786      +/-   ##
==========================================
+ Coverage   78.75%   78.76%   +0.01%     
==========================================
  Files          49       49              
  Lines        4909     4921      +12     
==========================================
+ Hits         3866     3876      +10     
- Misses       1043     1045       +2     

Files	Coverage Δ
edk2toolext/edk2_invocable.py	75.54% <ø> (+0.13%)	⬆️

... and 2 files with indirect coverage changes

[Javagedes]

@yangrongwei Thank you for your contribution :)