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GH-37061: [Docs][Format] Clarify semantics of GetSchema in FSQL (#38549)



### Rationale for this change

Schemas of result sets and bind parameters are ambiguous in a few cases when they interact.

### What changes are included in this PR?

Add documentation clarifying the expected behavior.

### Are these changes tested?
N/A

### Are there any user-facing changes?

No

* Closes: #37061

Authored-by: David Li <li.davidm96@gmail.com>
Signed-off-by: David Li <li.davidm96@gmail.com>