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GH-38528: [Python][Compute] Describe strptime format semantics (#38665)



### Rationale for this change

To prevent possible confusion with the compute strptime function, we now explicitly mention that the C/C++ semantics are followed.

### What changes are included in this PR?

The documentation of the `format` parameter of the `strptime` function is expanded.

### Are these changes tested?

N/A documentation change only.

### Are there any user-facing changes?

Just the documentation.
* Closes: #38528

Lead-authored-by: Dan Homola <dan.homola@hotmail.cz>
Co-authored-by: Dan Homola <dan.homola@gooddata.com>
Co-authored-by: Rok Mihevc <rok@mihevc.org>
Signed-off-by: Rok Mihevc <rok@mihevc.org>