docs: cleaner and more concise transfer function of Derivative()

[jaakkor2]

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional context

• Description of parameter x was inserted in Refactors Blocks module with @mtkmodel  #191 but in wrong position, move it down
• Clean up transfer function of Derivative. LHS is obtained from the implementation, the added RHS is simplified and the T->0 behavior is more obvious