Add -Wstar-is-type to the User's Guide

Documentation for -Wstar-is-type is missing in the 8.6.1 release.

Creating this ticket with a milestone as a reminder to cherry-pick the patch to 8.6.2 (right now it's merged to master).

Trac metadata

Trac field	Value
Version	8.6.1
Type	Task
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Documentation
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	bgamari
Operating system
Architecture

changed milestone to %8.6.2

changed weight to 5

added Ttask Trac import documentation labels

added 1 deleted label

closed

Merged to ghc-8.6 with 4ab2f347.

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedFixed

removed 1 deleted label

added Pnormal label