cmd/doc: show which version of Go introduced symbols

[FiloSottile]

#5778 introduced this feature in x/tools/cmd/godoc, but it is missing in cmd/doc, which replaces the godoc command line mode.

/cc @dmitshur

[agnivade]

Just a minor clarificaition: godoc CLI mode just shows the versions of fields in a struct, not functions or methods. The UI shows all. Are we going to show versions for all or just struct fields ?