[amarcozzi]

Closes #737

User-facing text pointed readers to docs/units.md, which is internal and not published. This drops the sentence everywhere it reached users. The rule itself (UDUNITS-2-conformant ASCII with ** exponents) is still stated in each place.

Changes

• grids/schema.py: Band.unit description
• grids/upload/geotiff/schema.py: UploadBandDefinition.unit description
• grids/upload/netcdf/router.py: endpoint docstring
• lib/units.py: the validate_unit error message, which reaches users as a 422 detail for a non-canonical unit. It now reads Unit 'kg/m^3' is not canonical; expected 'kg/m**3'.

The internal module docstring in lib/units.py keeps its reference.

Ripples

• The SDK's generated client copies these descriptions, so its next regen picks up the change. No code change is needed.
• No contract change: only description text and one error-message string.

Verification

• pytest tests/resources/grids/test_schema.py tests/resources/grids/upload (schema tests): 121 passed
• services/lib: pytest tests/test_units.py: 41 passed
• import api.main works