[C++] Exclude internal classes from documentation

[asfimport]

Our API docs are a bit smelly with quite a few internal classes and methods:

http://arrow.apache.org/docs/cpp/annotated.html

We should add internal constructs to an arrow::internal namespace so that we can use the EXCLUDE_SYMBOLS rule in Doxygen to exclude them from the documentation.

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

_{Note: This issue was originally created as ARROW-884. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
PR: #918

[asfimport]

Uwe Korn / @xhochy:
Issue resolved by pull request 918
#918