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MINOR: [C++][Python] [Docs] clearer description of q argument of quantiles (#37380)



### Rationale for this change
Documention of the `q` parameter of quantiles is made more precise.

### What changes are included in this PR?
Only documentation changes.

### Are these changes tested?
No

### Are there any user-facing changes?
No

Authored-by: Christian Lorentzen <lorentzen.ch@gmail.com>
Signed-off-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>