go spec: clarification re: method sets of newly declared pointer types

go spec: clarification re: method sets of newly declared pointer types

- added an example to Type declarations section clarifying the
  situation brought up with issue 1324
- slightly re-ordered paragraphs in Types section
- added separate heading for method set section and refer to it
  from elsewhere in the spec
- no language changes

R=rsc, r, iant, ken2, r2
CC=golang-dev
https://golang.org/cl/4145043