SpecConstr messages should be reworded, and not be shown at standard verbosity level

Currently GHC displays many messages about "specialisation over constructors" that typically look like this:

SpecConstr
    Function `$wks2{v s2dJ} [lid]'
      has one call pattern, but the limit is 0
    Use -fspec-constr-count=n to set the bound
    Use -dppr-debug to see specialisations

Some things are wrong with the design of this message:

(1.) It is not apparent from the message if it is a fatal error, warning, or debug information. (End users who install the Haskell platform often think that something has gone wrong.)

(2.) It is not apparent from the message that it is comes from the specialiser, unless you have read SpecConstr.lhs.

(3.) The message is shown at standard verbosity, but it probably shouldn't.

(4.) -fspec-constr-count is referenced, but not documented.

See also #4288.

Trac metadata

Trac field	Value
Version	7.0.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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