Edit: Implemented in #49757. From the docs:

This feature allows you to add alias(es) to an item when using the rustdoc search through the doc(alias) attribute. Example:

#![feature(doc_alias)]
#[doc(alias = "x")]
#[doc(alias = "big")]
pub struct BigX;

Then, when looking for it through the rustdoc search, if you enter "x" or "big", search will show the BigX struct first.