[Python] document differences w.r.t. fastparquet

[asfimport]

differences in options and/or actual written file formats w.r.t. https://fastparquet.readthedocs.io/en/latest/

• null handling
• non-supported type handling
• options that can be passed via top-level functions

Reporter: Jeff Reback / @jreback

_{Note: This issue was originally created as ARROW-760. Please see the migration documentation for further details.}

[asfimport]

Jeff Reback / @jreback:
xref dask/fastparquet#122
xref pandas-dev/pandas#15838

[asfimport]

Wes McKinney / @wesm:
@jreback I just moved this JIRA to the Arrow side – I think the differences mostly have to do with the Python API, but we can make a list of C++-only requirements that we need from parquet-cpp (e.g. an API in parquet_arrow for reading a single row group vs. the entire file)

[asfimport]

Jeff Reback / @jreback:
yep, was just writing the docs and wanted to list (or probably better), point to a document with any differences.

[asfimport]

Wes McKinney / @wesm:
I think it makes most sense to document this in the pandas docs where users may encounter differences between the libraries.

[asfimport]

Todd Farmer / @toddfarmer:
Transitioning issue from Resolved to Closed to based on resolution field value.