doc: updateProtPool and flexibilizeProtConcs

[edkerk]

Main improvements in this PR:

• doc:
  • more detailed explanation, particularly considering over-constraining proteomics-integrated models (solves #296)

I hereby confirm that I have:

• [x] Selected develop as a target branch (top left drop-down menu)

[github-actions[bot]]

This PR has been automatically tested with GH Actions. Here is the output of the tests:

 
Running geckoCoreFunctionTests
Done geckoCoreFunctionTests
__________

Note: In the case of multiple test runs, this post will be edited.