[Format][C++] Use two-part EOS and amend Format documentation

[asfimport]

Per mailing list discussion

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

Related issues:

• [Format] Tracking for ensuring flatbuffer serialized values are aligned in stream/files. (is a child of)

PRs and other links:

• GitHub Pull Request #5361

_{Note: This issue was originally created as ARROW-6541. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 5361
#5361