arrow

MINOR: [C++][Docs] Add \deprecated tag to deprecated BufferReader constructor (#38721)

### Rationale for this change

The PR [GH-37360](https://github.com/apache/arrow/pull/37360) for issue [GH-37212](https://github.com/apache/arrow/issues/37212) deprecated three BufferReader constructors and I noticed one of them was missing a `\deprecated` command.

### What changes are included in this PR?

This adds a `\deprecated` command one of the deprecated constructors with a message. The other two didn't need it because they weren't already documented. i.e., they weren't listed under https://arrow.apache.org/docs/cpp/api/io.html so documenting them at this point just to add `\deprecated` wouldn't make sense.

### Are these changes tested?

No, this is a simple docs change.

### Are there any user-facing changes?

Yes, this adds a notice to the docs for this particular method.

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