[Python] Indicate in NativeFile docstrings methods that are part of the RawIOBase API but not implemented

[asfimport]

see #2422

Reporter: Wes McKinney / @wesm
Assignee: Wes McKinney / @wesm

PRs and other links:

• GitHub Pull Request #2577

_{Note: This issue was originally created as ARROW-3056. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 2577
#2577