[DOCS-496] Clarify which commits must have coverage data to calculate each coverage metric

[prcr]

Type of feedback

• [ ] Bug or incorrect information
• [ ] Missing information
• [ ] Outdated information
• [X] Enhancement
• [ ] Other

Feedback

We should explicitly describe which commits must have coverage data so that Codacy can calculate:

• Coverage variation: both the head commit of the pull request branch and common ancestor commit must have coverage data
• Diff coverage: only the head commit of the pull request branch needs to have coverage data

Here's another way of explaining the associated logic:

A: Ancestor commit
H: Head commit

A & H =>  Codacy calculates the coverage variation and the diff coverage
!A & H => Codacy calculates diff coverage only (error on coverage variation)
A & !H => Codacy can't calculate any coverage metric (error on both coverage metrics)
!A & !H => Codacy can't calculate any coverage metric (error on both coverage metrics)

Affected pages This feedback applies to the following documentation pages / URLs:

https://docs.codacy.com/coverage-reporter/#validating-coverage

Additional context If applicable, provide any other context or information that could be useful in updating the documentation:

• This feedback was provided by @fjrdomingues after the insights from this Jira issue.
• See also this private Slack message

[github-actions[bot]]

Internal Jira issue: DOCS-496