rustdoc: Rename misleading function

- `is_associated` -> `is_type_alias`

`is_associated` is not a good name for what this is doing. If you look at
https://github.com/rust-lang/rust/pull/74489/files#diff-6a301d597807ee441a41e7237800563dR296,
is_associated() and as_assoc_kind() do completely different things, but
from the name it sounds like they're similar.

Author: jyn514

src/librustdoc/clean/types.rs

@@ -282,7 +282,7 @@ pub enum ItemEnum {
 }
 
 impl ItemEnum {
-    pub fn is_associated(&self) -> bool {
+    pub fn is_type_alias(&self) -> bool {
         match *self {
             ItemEnum::TypedefItem(_, _) | ItemEnum::AssocTypeItem(_, _) => true,
             _ => false,

src/librustdoc/html/render/mod.rs

@@ -3612,7 +3612,7 @@ fn render_impl(
         };
 
         let (is_hidden, extra_class) =
-            if (trait_.is_none() || item.doc_value().is_some() || item.inner.is_associated())
+            if (trait_.is_none() || item.doc_value().is_some() || item.inner.is_type_alias())
                 && !is_default_item
             {
                 (false, "")