Fix two indicator signature issues - clean up changelog

[aulemahal]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • This PR fixes The phase parameter of precip_accumulation is not included in the indicator docstring but discussed in the docstring #1763 and fixes xclim.atmos.growing_season_end gives operation ValueError and returns nan values #2056.
• Tests for the changes have been added (for bug fixes / features)
  • (If applicable) Documentation has been added / updated (for bug fixes / features)
• CHANGELOG.rst has been updated (with summary of main changes)
  • Link to issue (:issue:number) and pull request (:pull:number) has been added

What kind of change does this PR introduce?

• Fix overriden default value of op in growing_season_end.
• Remove thresh from precip_accumulation's signature.
• Clean up the changelog:
  • In "New indicators", point to indicators, not indices
  • Move mention of CO2 tracking to internal changes.
  • Move "Bug fixes" above "Internal changes"
  • Add precision to the mention of rv_continuous.

Does this PR introduce a breaking change?

Yes and no, the default argument of an indicator was changed. But the previous one was raising an error.

Other information:

[Zeitsperre]

Thanks for the changelog cleanup!