Mention `ClassLinker::LinkFieldsHelper::LinkFields` in ART's doc.

Replace mentions of `ClassLinker::LinkFields` with
`ClassLinker::LinkFieldsHelper::LinkFields` in ART's documentation.

Test: n/a (comment-only change)
Bug: 175869411
Change-Id: I9f74cad5d863d91dc0fc04e984350b1fc853d726
3 files changed