DOC: Clarify read_raw_nirx expects directory path, not file path

[natinew77-creator]

Summary

This PR improves the documentation for read_raw_nirx to prevent the common "OSError: Need a directory for fname but found a file..." error that users encounter when passing a single file path instead of a directory path.

Addresses #13353

Changes

mne/io/nirx/nirx.py

• Added a note in the read_raw_nirx docstring explaining the directory requirement
• Clarified that passing a file path will result in an error
• Added suggestion to use read_raw_snirf for .snirf files

tutorials/io/30_reading_fnirs_data.py

• Added an important note in the fNIRS tutorial with:
  • Correct vs incorrect usage examples
  • Troubleshooting guidance for the OSError
  • Alternative for SNIRF format files

Testing

Documentation-only change. No new tests required.This improves the documentation for read_raw_nirx to prevent the common 'OSError: Need a directory for fname but found a file' error that users encounter when passing a single file path instead of a directory path.

Changes:

• Added a note in the read_raw_nirx docstring explaining the directory requirement and suggesting read_raw_snirf for .snirf files
• Added an important note in the fNIRS tutorial with correct/incorrect examples and troubleshooting guidance

Fixes #13353

Reference issue (if any)

What does this implement/fix?

Additional information