go/doc, godoc: when filtering for godoc, don't remove elements of a declaration
go/doc, godoc: when filtering for godoc, don't remove elements of a declaration

Partially revert CL 4518050. In go/doc.go, instead of calling the go/ast filter
functions, implement the corresponding match functions that do no remove
declaration elements.

Fixes issue 1803.

R=rsc
CC=golang-dev
http://codereview.appspot.com/4517055

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang