When a conduit method requires a string constant, call it "string-const" not "enum"

Summary: Ref T5058. The use of "enum" is confusing; we mean "choose one of these specific string constants". Make this more clear.

Test Plan: Viewed each call from the web UI.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T5058

Differential Revision: https://secure.phabricator.com/D9127