docs(UcnFile): mention MODFLOW 6 GWT concentration file support

[TameTheGame]

Before: the binaryfile module summary lists UcnFile as the reader for MT3DMS concentration files, and its class notes describe "a MT3D binary head file". Nothing in the docs says UcnFile also reads MODFLOW 6 GWT concentration output, which it has done since #2718 (first released in 3.11.0). Someone who keeps only the output files, as in #2784, gets no hint that this works.

After: the module summary and class notes name both MT3DMS and MODFLOW 6 GWT concentration files, the notes say the file type is detected from the first record header, and the examples show opening a GWT file.

This is a docstring-only change in flopy/utils/binaryfile/init.py. test_mf6_concentration_build_index already covers reading a GWT file with UcnFile, so I didn't add a test.

Checked locally on Windows with Python 3.11: ruff check, ruff format --check and codespell pass, and the changed docstrings render through napoleon without warnings. test_binaryfile.py passes apart from the two test_read_mf2005_freyberg cases, which need an mf2005 executable I don't have installed and fail the same way without this change. I didn't run the full suite.

On #2784: with the snippet from the issue, 3.10.0 raises EOFError on the GWT test output (gwt_mst03.ucn) and 3.11.0 reads it, so the original report looks resolved by #2718. I've used "Refs" rather than "Closes" so you can decide whether to close it.

Refs #2784

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.8%. Comparing base (556c088) to head (5b180d9).
⚠️ Report is 243 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2865      +/-   ##
===========================================
+ Coverage     55.5%    73.8%   +18.2%     
===========================================
  Files          644      659      +15     
  Lines       124135   132511    +8376     
===========================================
+ Hits         68947    97812   +28865     
+ Misses       55188    34699   -20489     

Files with missing lines	Coverage Δ
flopy/utils/binaryfile/__init__.py	74.6% <ø> (-9.9%)	⬇️

... and 585 files with indirect coverage changes

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