Documentation fixes

Change-Id: I8359c402819ed475663385b3bd850f0ae209cdf4
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/18421
Reviewed-by: David Neto <dneto@google.com>

src/ast/literal.h

@@ -61,6 +61,7 @@ class Literal {
 
  protected:
   /// Constructor
+  /// @param type the type of the literal
   explicit Literal(ast::type::Type* type);
 
  private:

src/context.h

@@ -30,6 +30,7 @@ class Context {
   /// Resets the state of this context.
   void Reset();
 
+  /// @returns the Type Manager
   TypeManager& type_mgr() { return type_mgr_; }
 
  private:

src/writer/spirv/operand.h

@@ -50,7 +50,9 @@ class Operand {
   ~Operand();
 
   /// Copy assignment
-  Operand& operator=(const Operand&) = default;
+  /// @param b the operand to copy
+  /// @returns a copy of this operand
+  Operand& operator=(const Operand& b) = default;
 
   /// Sets the float value
   /// @param val the value to set