Fix missing Default: and optional markers in docstrings

[antonwolfy]

This PR cleans up the documentation, so that optional parameters consistently document their default values.
The changes are documentation-only — no functional/runtime behavior is affected.

The dpnp.tensor subtree is intentionally left out and will be addressed in a separate PR, since currently there is no documentation rendering for it.

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[coveralls]

coverage: 78.069%. remained the same — missing-default-optional-in-docstring into master

[github-actions]

Array API standard conformance tests for dpnp=0.21.0dev1=py313h509198e_22 ran successfully.
Passed: 1356
Failed: 4
Skipped: 16

[github-actions]

View rendered docs @ https://intelpython.github.io/dpnp/pull/2966/index.html