unify notation k-regular vs regular sequence in docstrings in corresponding module

[dkrenn]

Steps To Reproduce

No response

Expected Behavior

In the docstrings in the modules sage.combinat.k_regular_sequence there should either be consistently used one version of "k-regular sequence" or "regular sequence" be used, or at least a consistent rule when to use what be used (plus a comment at the beginning of the module.)

See the discussions #36011 (comment) and #36011 (review)

Actual Behavior

At the moment most of the time "k-regular sequence" is used and sometimes "regular sequence", but without a rule or system.

Additional Information

No response

Environment

- **Sage Version**: 10.0

Checklist

• I have searched the existing issues for a bug report that matches the one I want to file, without success.
• I have read the documentation and troubleshoot guide