Document function parameter defaults in GDScript reference

This also fixes some reStructuredText syntax issues.

tutorials/scripting/gdscript/gdscript_basics.rst

@@ -65,8 +65,8 @@ here's an example of how GDScript looks.
     var v3 = Vector3(1, 2, 3)
 
 
-    # Functions.
-    func some_function(param1, param2, param3):
+    # Function, with a default value for the last parameter.
+    func some_function(param1, param2, param3 = 123):
         const local_const = 5
 
         if param1 < local_const:
@@ -1411,6 +1411,20 @@ function's first argument, unlike Python).
 
 A function can ``return`` at any point. The default return value is ``null``.
 
+By default, all function parameters are required. You can make one or more
+parameters at the end optional by assigning a default value to them:
+
+::
+
+    # Since the last two parameters are optional, all these calls are valid:
+    # - my_function(1)
+    # - my_function(1, 20)
+    # - my_function(1, 20, 100)
+    func my_function(a_required, b_optional = 10, c_optional = 42):
+        print(a_required)
+        print(b_optional)
+        print(c_optional)
+
 If a function contains only one line of code, it can be written on one line:
 
 ::
@@ -2229,7 +2243,7 @@ abstract class:
     an abstract class to a node. If you attempt to do so, the engine will print
     an error when running the scene:
 
-    ::
+    .. code-block:: none
 
         Cannot set object script. Script '<path to script>' should not be abstract.
 
@@ -2351,7 +2365,7 @@ This is better explained through examples. Consider this scenario:
     var message = null
 
 
-    func _init(e=null):
+    func _init(e = null):
         entity = e
 
 
@@ -2363,7 +2377,7 @@ This is better explained through examples. Consider this scenario:
     extends "state.gd"
 
 
-    func _init(e=null, m=null):
+    func _init(e = null, m = null):
         super(e)
         # Do something with 'e'.
         message = m
@@ -2382,10 +2396,10 @@ There are a few things to keep in mind here:
 
    ::
 
-   # idle.gd
+       # idle.gd
 
-   func _init():
-       super(5)
+       func _init():
+           super(5)
 
 Static constructor
 ~~~~~~~~~~~~~~~~~~