(maint) establish the synonym "composite key" for uniqueness key

The uniqueness_key (as found in the Type class) is an awkward name for
what is similar to the concept of composite keys as found in DBMSs.
This alternative nomenclature was proposed by Henrik Lindberg.
(maint) Partial cleanup of the parameters/properties/attributes documentation

The concepts of properties vs. parameters (vs. metaparameters) and the
generic term "attributes" has been confused in various bits of documentation
of the Puppet::Type base class for resource types.

Clear up some of the comments by making appropriate use of these terms:
 - properties for attributes that are no parameters
 - parameters for attributes that are no properties
 - attributes if both are encompassed
Metaparameters are really just a special case of parameters. They should be
included when properties and parameters are enumerated instead of
just refering to "attributes".

It's also worth to note that since parameters and properties don't share
a common base class (Puppet::Attribute?), a method return or parameter type
that can be either cannot be made more specific than "Object", e.g.

  return [Object] the named attribute