Update documentation regarding SpecConstr.

 * Note new SPEC type in release notes.
 * Document SPEC in the users guide under the documentation for
   -fspec-constr.
 * Clean up comments in SpecConstr regarding the forcing of
   specialisation (see Note [Forcing specialisation].)

Signed-off-by: Austin Seipp <austin@well-typed.com>
Release note blurbs for directory.

Signed-off-by: Austin Seipp <austin@well-typed.com>