Boolean format in Plain Decoder

In the encoding.md document is written that the plain encoder for boolean uses [RLE/BitPacking](https://github.com/apache/parquet-format/blob/master/Encodings.md#plain-plain--0)

While in the cpp implementation seems is just using [simple bit decoding back to back.](https://github.com/apache/parquet-cpp/blob/master/src/parquet/encodings/plain-encoding.h#L151)

Which one is the right format ?

**Reporter**: [Fabrizio Milo](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=fmilo)

_{**Note**: *This issue was originally created as [PARQUET-574](https://issues.apache.org/jira/browse/PARQUET-574). Please see the [migration documentation](https://issues.apache.org/jira/browse/PARQUET-2502) for further details.*}

Start with encoding.md and the linked Encodings.md section describing boolean PLAIN encoding, then compare it with parquet-cpp/src/parquet/encodings/plain-encoding.h around line 151. Determine which representation is correct and update the documentation or implementation accordingly, with the specification and behavior consistent when done.

Written by the indexing model from the issue text.