spec: clean up intro and replace bolded text with prose

- Remove "Draft" disclaimer. We're not done but the spec
  is in usable shape with respect to generics features.

- Remove section on "Earlier version" and fold information
  into the "Intro" section.

- Remove caveat for shifts: the rules for arithmetic operators
  on type parameters apply for them as well.

- Simply state that we don't support arguments of type parameter
  type for the built-ins real, imag, and complex.

Fixes #51182.

Change-Id: I6df1427de685cfe7055b64e91753aa7ebff70565
Reviewed-on: https://go-review.googlesource.com/c/go/+/391695


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>