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MINOR: [FlightRPC][C++][Python][Docs] Minor tweaks to docstrings (#36900)

### Rationale for this change

Improves language surrounding Flight Shutdown/Wait/Serve.

### What changes are included in this PR?

Just docstring tweaks.

### Are these changes tested?

No tests, just docstrings.

### Are there any user-facing changes?

n/a

Authored-by: Bryce Mecum <petridish@gmail.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>