refactor: update docs and handling of zero stride of `blas/ext/base/dapxsumkbn`

[headlessNode]

Description

What is the purpose of this pull request?

This pull request:

• update documentation
• update handling of case when stride is equal to zero.
• update tests according to above change.

Related Issues

Does this pull request have any related issues?

No.

Questions

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No.

Other

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No.

Checklist

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• [x] Read, understood, and followed the contributing guidelines.

• • *

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[stdlib-bot]

Coverage Report

Package	Statements	Branches	Functions	Lines
blas/ext/base/dapxsumkbn	$\color{green}392/392$ $\color{green}+100.00\%$	$\color{green}21/21$ $\color{green}+100.00\%$	$\color{green}4/4$ $\color{green}+100.00\%$	$\color{green}392/392$ $\color{green}+100.00\%$

The above coverage report was generated for the changes in this PR.

[headlessNode]

@kgryte The test failure here is from a downstream test suite. I'll update it once this PR is merged.

[kgryte]

@headlessNode This is already merged, but I just saw that we are still using fabs in the implementation. We can migrate to using @stdlib/math/base/special/abs.