Expression spec: Add clarification for String compare

Relational and equality operators require both operands to be of
the same type.
Treat the string 'A' and "A" as same type, but for "A" and L"A"
are not same type since one is general string, another is unicode
string.

Cc: Liming Gao <[email protected]>
Cc: Michael Kinney <[email protected]>
Cc: Kevin W Shaw <[email protected]>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Yonghong Zhu <[email protected]>
Reviewed-by: Liming Gao <[email protected]>

Author: Yonghong Zhu

2_expression_overview.md

@@ -1,7 +1,7 @@
 <!--- @file
   2 Expression Overview
 
-  Copyright (c) 2014-2017, Intel Corporation. All rights reserved.<BR>
+  Copyright (c) 2014-2018, Intel Corporation. All rights reserved.<BR>
 
   Redistribution and use in source (original document form) and 'compiled'
   forms (converted to PDF, epub, HTML and other formats) with or without
@@ -83,10 +83,13 @@ directives section do not apply.
     arithmetic in type.
 
 11. Relational and equality operators require both operands to be of the same
-    type. Relational comparison on string literals and byte arrays must be
-    performed comparing the byte values, left to right. The first pair of bytes
-    (or a single extra byte) that is not equal will determine the result of the
-    comparison. The following are examples of string comparisons:
+    type. The single quote string and double quote string are same type as
+    general string, while the string start with `L` character is the unicode
+    string. Unicode string should not compare with general string. Relational
+    comparison on string literals and byte arrays must be performed comparing
+    the byte values, left to right. The first pair of bytes (or a single extra
+    byte) that is not equal will determine the result of the comparison. The
+    following are examples of string comparisons:
     ```
     Foo = "zero", Bar = "three";
     ```