auto merge of #18528 : seanjensengrey/rust/doc-18498, r=brson

This addresses rust-lang/rust#18498 by adding a prepopulated search box to do site search on `doc.rust-lang.org` using duckduckgo AND generating a search url against the rust documentation using the internal search facilities.

* https://duckduckgo.com/?q=type+Option+unwrap_or_else+site%3Adoc.rust-lang.org
* http://doc.rust-lang.org/core/?search=unwrap_or_else