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MINOR: [Docs][Python] Document type aliasing in pa.field/pa.schema (#44512)

### Rationale for this change

PyArrow supports a set of type aliases, e.g., "string" aliases to pa.string() and these type aliases are triggered in calls to `pa.field` and `pa.schema`. Prior to this change, these weren't documented.

Note: I didn't think we wanted to deprecate these but if any reviewers want to discuss that let me know. The R package doesn't support a similar aliasing mechanism.

### What changes are included in this PR?

Updates to docs. One regression test.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

Better docs.

Authored-by: Bryce Mecum <petridish@gmail.com>
Signed-off-by: Bryce Mecum <petridish@gmail.com>