rustdoc does not generate documentation for extern blocks.

[pcwalton]

Yuck.

[thestinger]

Nominating for the feature complete milestone.

[emberian]

rustdoc_ng doesn't look into extern blocks. Should it?

[emberian]

#8125

[emberian]

cc @pcwalton

[alexcrichton]

Renaming bug with a more appropriate title now that rustdoc_ng is generating docs. The old title was:

"rustdoc thinks that extern blocks are modules named foreign_mod"