cmd/doc: doesn't expose builtin functions

[cespare]

go doc doesn't work on the functions in the pseudo-package builtin (probably because they don't look like exported names), even though they are shown by godoc.

Compare https://golang.org/pkg/builtin/#copy with

$ go doc builtin.copy
doc: symbol copy not present in package builtin installed in "builtin"

[robpike]

This is fixed at tip. See #12541.

[cespare]

Apologies, my github-search-fu was weak.