docs: improve doctests for ndarray instances in `blas/ext/base/ndarray/gcusumkbn` #25336

Job	Run time
Manage labels	0s
Auto-fix lint errors	0s
Rebase this PR on top of develop branch	0s
Update copyright header years	0s
Remove in-progress label	0s
Merge changes from develop branch into this PR	0s
Check for required files	-6s
	-6s

Provide feedback