Update docstring description of axis keyword in cumulative functions

[antonwolfy]

The axis keyword in cumulative functions supported only as an integer. No support of a axis with tuple value is assumed.
The PR proposes to update the docstrings description to reflect the actual behavior.

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[oleksandr-pavlyk]

Thank you @antonwolfy !

[github-actions]

Deleted rendered PR docs from intelpython.github.com/dpctl, latest should be updated shortly. 🤞

[coveralls]

coverage: 87.892%. remained the same
when pulling 05426b5 on update-axis-docstring-in-accumulate-functions
into 1243edc on master.

[github-actions]

Array API standard conformance tests for dpctl=0.17.0dev0=py310h15de555_333 ran successfully.
Passed: 883
Failed: 9
Skipped: 92