Update documentation for partial type signatures

In the documentation for partial type signatures, I found several bits that are out of date:

• There are several sample error messages given. These have been substantively reworded (Found hole _ with type: ... to Found type wildcard _ standing for ...) where I find the new wording much clearer.

• In the "Where can they occur?" section, there's no mention of TypeApplications, which is a new context where they can appear.

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Documentation
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

changed weight to 5

added Tbug Trac import documentation labels

assigned to @trac-thomasw

Trac metadata

Trac field	Value
Differential revisions	- → D2413

mentioned in commit 627c767b

changed milestone to %8.0.2

Thanks thomasw!

added 1 deleted label

closed

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedFixed

removed 1 deleted label

added Pnormal label