Clarify that {self} imports require a module parent

The `self` keyword in a use brace (e.g., `use m::{self}`) creates a
binding for the parent entity.  The note in `items.use.self.intro`
says that `self` "means the current module of the parent segment", but
there's no rule restricting what the parent can be.

The parent path must resolve to a module, enumeration, or trait --
i.e., entities that act as modules for name resolution.  Using `self`
with other entities such as structs or unions is rejected by the
compiler (as of rust-lang/rust#152996).

Let's add a rule to make this restriction explicit.

Author: traviscross

src/items/use-declarations.md

@@ -203,6 +203,26 @@ mod example {
 > [!NOTE]
 > `self` may also be used as the first segment of a path. The usage of `self` as the first segment and inside a `use` brace is logically the same; it means the current module of the parent segment, or the current module if there is no parent segment. See [`self`] in the paths chapter for more information on the meaning of a leading `self`.
 
+r[items.use.self.module]
+When `self` is used within [brace syntax], the path preceding the brace group must resolve to a [module], [enumeration], or [trait].
+
+```rust
+mod m {
+    pub enum E { V1, V2 }
+    pub trait Tr { fn f(&self); }
+}
+use m::{self as _}; // OK: Modules can be parents of `self`.
+use m::E::{self, V1}; // OK: Enums can be parents of `self`.
+use m::Tr::{self}; // OK: Traits can be parents of `self`.
+# fn main() {}
+```
+
+```rust,compile_fail,E0432
+struct S {}
+use S::{self as _}; // ERROR: Structs cannot be parents of `self`.
+# fn main() {}
+```
+
 r[items.use.self.namespace]
 `self` only creates a binding from the [type namespace] of the parent entity. For example, in the following, only the `foo` mod is imported:
 
@@ -433,9 +453,11 @@ r[items.use.restrictions.variant]
 
 [`$crate`]: paths.qualifiers.macro-crate
 [Attributes]: ../attributes.md
+[brace syntax]: items.use.multiple-syntax
 [Built-in types]: ../types.md
 [Derive macros]: macro.proc.derive
 [Enum variants]: enumerations.md
+[enumeration]: items.enum
 [`extern crate`]: extern-crates.md
 [`macro_rules`]: ../macros-by-example.md
 [`self`]: ../paths.md#self
@@ -444,10 +466,12 @@ r[items.use.restrictions.variant]
 [generic parameters]: generics.md
 [items]: ../items.md
 [local variables]: ../variables.md
+[module]: items.mod
 [name resolution ambiguities]: names.resolution.expansion.imports.ambiguity
 [namespace]: ../names/namespaces.md
 [namespaces]: ../names/namespaces.md
 [paths]: ../paths.md
 [tool attributes]: ../attributes.md#tool-attributes
+[trait]: items.traits
 [type alias]: type-aliases.md
 [type namespace]: ../names/namespaces.md