Rollup merge of #73567 - adetaylor:extern-doc-fix, r=dtolnay
Rollup merge of #73567 - adetaylor:extern-doc-fix, r=dtolnay

Clarify --extern documentation.

Fixes #64731, #73531.

See also https://github.com/rust-lang/rust/issues/64402#issuecomment-530852886

graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust