Improve docs around waveforms

[rly]

Motivation

It is confusing to write Units waveforms with different numbers of electrodes and units. This PR improves the API docs for Units.add_unit.

How to test the behavior?

make clean && make html

Checklist

• Did you update CHANGELOG.md with your changes?
• Have you checked our Contributing document?
• Have you ensured the PR clearly describes the problem and the solution?
• Is your contribution compliant with our coding style? This can be checked running ruff check . && codespell from the source directory.
• Have you checked to ensure that there aren't other open Pull Requests for the same change?
• Have you included the relevant issue number using "Fix #XXX" notation where XXX is the issue number? By including "Fix #XXX" you allow GitHub to close issue #XXX when the PR is merged.

[rly]

cc @khl02007

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.20%. Comparing base (fcd7008) to head (da0e55a).

Additional details and impacted files

@@           Coverage Diff           @@
##              dev    #1936   +/-   ##
=======================================
  Coverage   92.20%   92.20%           
=======================================
  Files          27       27           
  Lines        2656     2656           
  Branches      693      693           
=======================================
  Hits         2449     2449           
  Misses        134      134           
  Partials       73       73           

Flag	Coverage Δ
integration	73.15% <100.00%> (ø)
unit	83.77% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.