spec: more adjustments/corrections

- Change section title from "Type parameters lists" to
  "Type parameter declarations" as the enclosing section
  is about declarations.

- Correct section on parsing ambiguity in type parameter
  lists.

- Rephrase paragraphs on type parameters for method receivers
  and adjust examples.

- Remove duplicate prose in section on function argument type
  inference.

- Clarified "after substitution" column in Instantiations section.

Change-Id: Id76be9804ad96a3f1221e5c4942552dde015dfcb
Reviewed-on: https://go-review.googlesource.com/c/go/+/390994


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