#[doc(alias)]'s doc: say that ASCII spaces are allowed

Co-authored-by: Trevor Gross <[email protected]>

src/doc/rustdoc/src/advanced-features.md

@@ -80,7 +80,8 @@ 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.
 
-There are some limitations on the doc alias names though: you can't use `"` or whitespace.
+There are some limitations on the doc alias names though: they cannot contain quotes (`'`, `"`)
+or most whitespace. ASCII space is allowed if it does not start or end the alias.
 
 You can add multiple aliases at the same time by using a list: