rustdoc: Document is_assoc_ty()

It's adapted from the old documentation for the `is_generic` field.

src/librustdoc/clean/types.rs

@@ -1498,6 +1498,7 @@ impl Type {
         }
     }
 
+    /// Checks if this is a `T::Name` path for an associated type.
     crate fn is_assoc_ty(&self) -> bool {
         match self {
             ResolvedPath { path, .. } => path.is_assoc_ty(),
@@ -1990,6 +1991,7 @@ impl Path {
             + &self.segments.iter().map(|s| s.name.to_string()).collect::<Vec<_>>().join("::")
     }
 
+    /// Checks if this is a `T::Name` path for an associated type.
     crate fn is_assoc_ty(&self) -> bool {
         match self.res {
             Res::SelfTy(..) if self.segments.len() != 1 => true,