DOC: Make it clearer that count comes from astropy

[pllim]

Description

This pull request is to address confusion on where the unit of count comes from.

[github-actions]

Thank you for your contribution! 🌌 This checklist is meant to remind the package maintainers who will review this pull request of some common things to look for.

• Do the proposed changes actually accomplish desired goals?
• Do the proposed changes follow the STScI coding guidelines?
• Are tests added/updated as required? If so, do they follow the STScI testing guidelines?
• Are docs added/updated as required?
• Is rebase and/or squash necessary? If so, please provide the author with appropriate instructions.
• Did the CI pass? If no, are the failures related?
• Is a change log needed?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.14%. Comparing base (b89db64) to head (77e99e9).
⚠️ Report is 22 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #428   +/-   ##
=======================================
  Coverage   97.14%   97.14%           
=======================================
  Files          18       18           
  Lines        2035     2035           
=======================================
  Hits         1977     1977           
  Misses         58       58           

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

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[jryon]

Looks good to me. Thanks!