Document the name parameter in perlin() docstring

[brendancol]

Closes #3465

perlin() accepts a name parameter that sets the output DataArray name, but the docstring's Parameters section never mentioned it. The sibling generate_terrain() documents the same parameter, so this aligns the two.

• Add a name : str, default='perlin' entry to the perlin() docstring, matching the generate_terrain style.
• Add tests: name default and custom value are honored, and the docstring documents the parameter.

Docstring only. No signature change, no behavior change. Backends unaffected (numpy / cupy / dask+numpy / dask+cupy).

Test plan:

• pytest xrspatial/tests/test_perlin.py (14 passed, GPU paths included on a CUDA host)

[brendancol]

Domain-aware review (API-consistency sweep, perlin module).

Scope check: the diff is exactly the docstring entry plus two focused tests. No signature, behavior, or backend-dispatch changes. The name wording and default='perlin' match the sibling generate_terrain() convention, which is the consistency target.

Blockers: none.

Suggestions: none. test_perlin_name_param covers default and custom values on the numpy path; the name plumbing in perlin() is backend-agnostic (set on the wrapping DataArray after dispatch), so a numpy-only assertion is sufficient and a per-backend matrix would add no signal.

Nits: none.

Verified: pytest xrspatial/tests/test_perlin.py passes 14/14 including the GPU paths on a CUDA host.