Update doc alias documentation

Author: GuillaumeGomez

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

@@ -46,3 +46,5 @@ 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.