Clarify encoding schemes for boolean types

[asfimport]

In the Parquet format specification, under the section for Plain encoding, boolean is encoded using the deprecated bit-packed encoding. However, the section for bit-packed encoding specifies that it is only used for repetition/definition levels. This seems contradictory.

The section for RLE/bit-packed hybrid encoding 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 / @sunchao
Assignee: Chao Sun / @sunchao

_{Note: This issue was originally created as PARQUET-1249. Please see the migration documentation for further details.}

[asfimport]

Chao Sun / @sunchao:
Trying to make my first contribution here. Can someone add me as contributor? cc @rdblue, @wesm, @xhochy

[asfimport]

Wes McKinney / @wesm:
Added you

[asfimport]

Chao Sun / @sunchao:
Thanks!