Fix comma splice.

Remove comma before "that".
Use comma for introductory clauses.

Author: XP1

doc/en/core/undefined.md

@@ -7,7 +7,7 @@ being `undefined`.
 
 `undefined` is a type with exactly one value: `undefined`.
 
-The language also defines a global variable that has the value of `undefined`,
+The language also defines a global variable that has the value of `undefined`;
 this variable is also called `undefined`. However, this variable is **neither** a constant
 nor a keyword of the language. This means that its *value* can be easily 
 overwritten.
@@ -36,7 +36,7 @@ necessary to retrieve the value of `undefined` first.
 
 In order to protect code against a possible overwritten `undefined` variable, a 
 common technique used is to add an additional parameter to an
-[anonymous wrapper](#function.scopes), that gets no argument passed to it.
+[anonymous wrapper](#function.scopes) that gets no argument passed to it.
 
     var undefined = 123;
     (function(something, foo, undefined) {
@@ -55,8 +55,8 @@ wrapper.
 
     })('Hello World', 42);
 
-The only difference being here, that this version results in 4 more bytes being
-used in case it is minified and there is no other `var` statement inside the
+The only difference here is that this version results in 4 more bytes being
+used in case it is minified, and there is no other `var` statement inside the
 anonymous wrapper.
 
 ### Uses of `null`
@@ -66,7 +66,7 @@ the sense of a traditional *null*, the actual `null` (both a literal and a type)
 is more or less just another data type.
 
 It is used in some JavaScript internals (like declaring the end of the
-prototype chain by setting `Foo.prototype = null`), but in almost all cases it
+prototype chain by setting `Foo.prototype = null`), but in almost all cases, it
 can be replaced by `undefined`.