This slurps up everything inside of an 'extern' block into the enclosing module
in order to document them. The documentation must be on the items themselves,
and they'll show up next to everything else on the module index pages.
Closes#5953
Removes old rustdoc, moves rustdoc_ng into its place instead (plus drops the _ng
suffix). Also shreds all reference to rustdoc_ng from the Makefile rules.