Clarify encoding schemes for boolean types

In the Parquet format specification, under [the section for Plain encoding](https://github.com/apache/parquet-format/blob/master/Encodings.md#plain-plain--0), boolean is encoded using the deprecated bit-packed encoding. However, [the section for bit-packed encoding](https://github.com/apache/parquet-format/blob/master/Encodings.md#bit-packed-deprecated-bit_packed--4) specifies that it is only used for repetition/definition levels. This seems contradictory.

[The section for RLE/bit-packed hybrid encoding](https://github.com/apache/parquet-format/blob/master/Encodings.md#run-length-encoding--bit-packing-hybrid-rle--3) says "_Boolean values in data pages, as an alternative to PLAIN encoding_" - perhaps we should be specific and indicate this is only used for data page V2?

Also, implementation-wise, I saw parquet-cpp still encode boolean as plain 1-bit value while parquet-mr uses bit-packed encoding as described in the specification. Perhaps consolidation should be done for this.

**Reporter**: [Chao Sun](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=csun) / @sunchao
**Assignee**: [Chao Sun](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=csun) / @sunchao

_{**Note**: *This issue was originally created as [PARQUET-1249](https://issues.apache.org/jira/browse/PARQUET-1249). Please see the [migration documentation](https://issues.apache.org/jira/browse/PARQUET-2502) for further details.*}

Start with Encodings.md, especially the Plain, bit-packed, and RLE/bit-packed hybrid sections linked in the issue. Compare the specification wording with the boolean encoding behavior in parquet-cpp and parquet-mr. Done means the boolean encoding rules are consistent, including any data page V2 distinction, and the specification no longer appears contradictory.

Written by the indexing model from the issue text.