Fix AvoidDefaultValueForMandatoryParameter documentation, rule and tests

[bergmeister]

PR Summary

Fixed #876

• The documentation was not updated when the PSProvideDefaultParameterValue was changed to PSAvoidDefaultValueForMandatoryParameter (see issue)
• Fix Implementation not alwasy warn, even when not using CmdletBinding
• Existing tests were questionable -> complete refactoring

PR Checklist

Note: Tick the boxes below that apply to this pull request by putting an x between the square brackets. Please mark anything not applicable to this PR NA.

• PR has a meaningful title
  • [ x Use the present tense and imperative mood when describing your changes
• Summarized changes
• User facing documentation needed
• Change is not breaking. There will be warnings (by design) where there were not warnings but this is by design and therefore not a breaking change.
• Make sure you've added a new test if existing tests do not effectively test the code changed
• This PR is ready to merge and is not work in progress
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[LaurentDardenne]

The changelog.md file has not been modified, will it be done later?

[bergmeister]

I think the changelog gets only updated with all PRs at release time (which should hopefully be soon)