[spec] Tweak docs on default arguments (#3621)

Minor tweaks.
Add link to function type aliases.

Author: ntrel

spec/declaration.dd

@@ -411,18 +411,20 @@ assert(S.j == 4);
 $(H3 $(LNAME2 alias-function, Aliasing a Function Type))
 
         $(P Function types can be aliased:)
+
+$(SPEC_RUNNABLE_EXAMPLE_COMPILE
 ---
 alias Fun = int(string p);
-int fun(string){return 0;}
+int fun(string) {return 0;}
 static assert(is(typeof(fun) == Fun));
 
 alias MemberFun1 = int() const;
 alias MemberFun2 = const int();
 // leading attributes apply to the func, not the return type
 static assert(is(MemberFun1 == MemberFun2));
 --------------------
-
-        $(P Aliases can be used to call a function with different default
+)
+        $(P Type aliases can be used to call a function with different default
         arguments, change an argument from required to default or vice versa:)
 
 $(SPEC_RUNNABLE_EXAMPLE_RUN
@@ -434,7 +436,7 @@ void main() {
     foo();              // prints v: 6
     foo(8);             // prints v: 8
     Bar bar = &barbar;
-    // bar(4);             // compilation error, because the `Bar` alias
+    //bar(4);           // compilation error, because the `Bar` alias
                         // requires an explicit 2nd argument
     barbar(4);          // prints a: 4, b: 6, c: 7
     bar(4, 5);          // prints a: 4, b: 5, c: 9

spec/function.dd

@@ -1646,7 +1646,7 @@ void main()
 
         $(P See Also: $(RELATIVE_LINK2 lazy_variadic_functions, Lazy Variadic Functions))
 
-$(H3 $(LNAME2 function-default-args, Function Default Arguments))
+$(H3 $(LNAME2 function-default-args, Default Arguments))
 
         $(P Function parameter declarations can have default values:)
 
@@ -1664,14 +1664,14 @@ foo(4);   // same as foo(4, 3);
 ---
 module m;
 private immutable int b;
-pure void g(int a=b){}
+pure void g(int a = b) {}
 ---
 ---
 import m;
 int b;
 pure void f()
 {
-  g();  // ok, uses m.b
+    g();  // ok, uses m.b
 }
 ---
 
@@ -1680,21 +1680,25 @@ pure void f()
 ---
 module m;
 int b;
-pure void g(int a=b){}
+pure void g(int a = b) {}
 ---
 ---
 import m;
 enum int b = 3;
 pure void f()
 {
-  g();  // error, cannot access mutable global `m.b` in pure function
+    g();  // error, cannot access mutable global `m.b` in pure function
 }
 ---
 
         $(P If the default value for a parameter is given, all following
         parameters must also have default values.
         )
 
+        $(P See also: function type aliases
+        $(DDSUBLINK spec/declaration, alias-function, with default values).)
+
+
 $(H3 $(LNAME2 return-ref-parameters, Return Ref Parameters))
 
         $(P Return ref parameters are used with