spec: move Method expr/value section near selectors

Preparation for fixing issue 5769 (method selectors
do not auto-dereference): The actual fix may require
some cleanups in all these sections, and syntactically,
method expressions and method values are selector
expressions. Moving them next to each other so that
it's easy to see the actual changes (next CL).

No content changes besides the section moves.

LGTM=iant, rsc
R=r, rsc, iant, ken
CC=golang-codereviews
https://golang.org/cl/132300043