doc: update release notes for 2.18.2

[jhamman]

Release notes for a small patch update fixing an issue with the experimental v3 compressor codepath.

TODO:

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/tutorial.rst
• Changes documented in docs/release.rst
• GitHub Actions have all passed
• Test coverage is 100% (Codecov passes)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.98%. Comparing base (3e98c3b) to head (847c66c).
Report is 11 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1915   +/-   ##
=======================================
  Coverage   99.98%   99.98%           
=======================================
  Files          38       38           
  Lines       14703    14703           
=======================================
  Hits        14701    14701           
  Misses          2        2