Clarify that the type of dictionary encoded field should be the encoded(index) type

[asfimport]

Reporter: Li Jin / @icexelloss

PRs and other links:

• GitHub Pull Request #2798

_{Note: This issue was originally created as ARROW-3566. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 2798
#2798