spec: explicitly define integer, floating-point, and complex types

The terms "integer type", "floating-point type", and "complex type"
are used frequently in the spec but are not explicitly (only indirectly)
defined.

Slightly rephrased the section on numeric types and introduce these
terms explicitly. Add links to this section.

Change-Id: I3fb888933bece047da8b356b684c855618e9aee4
Reviewed-on: https://go-review.googlesource.com/c/go/+/384157


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