Separating Method from Func made the code only more complicated
without adding much to the useability/readability of the API.
Reverted to where it was, but leaving the new method-specific
fields Orig and Level.

Former clients (godoc) of doc.Method only used the Func fields;
and because Func was embedded, no changes are needed with respect
to the removal of Method.

Changed type of Func.Recv from ast.Expr to string. This was a
long-standing TODO. Also implemented Func.Orig field (another TODO).

No further go/doc API changes are expected for Go 1.

R=rsc, r, r
CC=golang-dev
https://golang.org/cl/5577043