Update docs to highlight lambda comparison

spec/traits.dd

@@ -1020,6 +1020,68 @@ void main()
         $(P If the two arguments are expressions made up of literals
         or enums that evaluate to the same value, true is returned.)
 
+        $(P If the two arguments are both lambda functions (or aliases
+        to lambda functions), then they are compared for equality. For
+        the comparison to be computed correctly, the following conditions
+        must be met for both lambda functions:)
+
+        $(OL
+        $(LI The lambda function arguments must not have a template
+        instantiation as an explicit argument type. Any other argument
+        types (basic, user-defined, template) are supported.)
+        $(LI The lambda function body must contain a single expression
+        (no return statement) which contains only numeric values,
+        manifest constants, enum values and function arguments. If the
+        expression contains local variables, function calls or return
+        statements, the function is considered uncomparable.)
+        )
+
+        $(P If these constraints aren't fulfilled, the function is considered
+        incomparable and `isSame` returns $(D false).)
+
+$(SPEC_RUNNABLE_EXAMPLE_COMPILE
+---
+void main()
+{
+    static assert(__traits(isSame, (a, b) => a + b, (c, d) => c + d));
+    static assert(__traits(isSame, a => ++a, b => ++b));
+    static assert(!__traits(isSame, (int a, int b) => a + b, (a, b) => a + b));
+    static assert(__traits(isSame, (a, b) => a + b + 10, (c, d) => c + d + 10));
+
+    // lambdas accessing local variables are considered incomparable
+    int b;
+    static assert(!__traits(isSame, a => a + b, a => a + b));
+
+    // lambdas calling a function inside their body are considered incomparable
+    int f() { return 3;}
+    static assert(!__traits(isSame, a => a + f(), a => a + f()));
+
+    class A
+    {
+        int a;
+        this(int a)
+        {
+            this.a = a;
+        }
+    }
+
+    class B
+    {
+        int a;
+        this(int a)
+        {
+            this.a = a;
+        }
+    }
+
+    static assert(__traits(isSame, (A a) => ++a.a, (A b) => ++b.a));
+    // lambdas with different data types are considered incomparable,
+    // even if the memory layout is the same
+    static assert(!__traits(isSame, (A a) => ++a.a, (B a) => ++a.a));
+}
+---
+)
+
 $(H2 $(GNAME compiles))
 
         $(P Returns a bool $(D true) if all of the arguments