[#250] fix documentation around rejected proposal slash value

[pasqu4le]

Description

Problem: the documentation wasn't fully updated to reflect the behavior of the rejected proposal slash value lambda, the last time it was changed. As a result the docs contradict each other in some cases and don't match the actual behavior.

Solution: fix the various issues in the docs related to this lambda and its usage.

Related issue(s)

Fixup for #250 (and probably older issues as well)

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Related changes (conditional)

• Tests

  • [x] If I added new functionality, I added tests covering it.
  • [x] If I fixed a bug, I added a regression test to prevent the bug from silently reappearing again.

• Documentation

  • [x] I checked whether I should update the docs and did so if necessary:
  • README
  • Haddock

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• [x] My commits comply with the following policy.
• [x] My code complies with the style guide.