Pkg docs: fixes for proper display via help()

* Removed the Pkg prefix in the definitions, since otherwise
  help() doesn't work
* Added preamble to make clear Pkg functions are not exported
* Linked the page in stdlib index
* Updated helpdb.jl

Together with the actual writing of the docs in
468c0297f67e16a2d47983c2a7b31a3bcd30daed, this
closes #4696.