fix: salinity units for observation converters WOD and GTSPP an MOM6

[hkershaw-brown]

Description:

Added notes/warning for the unit change by the observation converters. WOD, GTSPP

MOM6 convert salinity to MSU in model_interpolate

Fixes issue

Fixes #509

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [x] Documentation update

Documentation changes needed?

• [x] My change requires a change to the documentation.
  • [x] I have updated the documentation accordingly.

Tests

Please describe any tests you ran to verify your changes. Documentation build.

Checklist for merging

• [ ] Updated changelog entry
• [ ] Documentation updated
• [ ] Update conf.py

Checklist for release

• [ ] Merge into main
• [ ] Create release from the main branch with appropriate tag
• [ ] Delete feature-branch

Testing Datasets

• [x] Dataset needed for testing available upon request
• [ ] Dataset download instructions included
• [ ] No dataset needed