invocables: Update --help information

[Javagedes]

1. If a configuration file is provided, will provide all available options for architecture (-a), package (-p) and target (-t).

2. Ensures that the CLI env guidance is provided to the user even If no configuration file is provided (bugfix).

3. Updates some general help text to be more clear.

[codecov]

Codecov Report

Attention: Patch coverage is 62.96296% with 10 lines in your changes are missing coverage. Please review.

Project coverage is 78.82%. Comparing base (bf643b0) to head (475ceeb).
Report is 27 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #754      +/-   ##
==========================================
+ Coverage   78.75%   78.82%   +0.07%     
==========================================
  Files          49       49              
  Lines        4909     4935      +26     
==========================================
+ Hits         3866     3890      +24     
- Misses       1043     1045       +2     

Files	Coverage Δ
...oolext/invocables/edk2_multipkg_aware_invocable.py	83.63% <100.00%> (+2.38%)	⬆️
edk2toolext/edk2_invocable.py	75.93% <80.00%> (+0.52%)	⬆️
edk2toolext/invocables/edk2_platform_build.py	73.80% <40.00%> (+1.30%)	⬆️

... and 2 files with indirect coverage changes