Fix and improve Unicode escape sequence info (C#)

docs/csharp/programming-guide/strings/index.md

@@ -1,7 +1,7 @@
 ---
 title: "Strings - C# Programming Guide"
 ms.custom: seodec18
-ms.date: 07/20/2015
+ms.date: 06/27/2019
 helpviewer_keywords: 
   - "C# language, strings"
   - "strings [C#]"
@@ -56,13 +56,16 @@ A string is an object of type <xref:System.String> whose value is text. Internal
 |\n|New line|0x000A|  
 |\r|Carriage return|0x000D|  
 |\t|Horizontal tab|0x0009|  
-|\U|Unicode escape sequence for surrogate pairs.|\Unnnnnnnn|  
-|\u|Unicode escape sequence|\u0041 = "A"|  
+|\U|Unicode escape sequence (UTF-32)|`\U00nnnnnn` (e.g. `\U0001F47D` = "&#x1F47D;")|  
+|\u|Unicode escape sequence (UTF-16)|`\unnnn` (e.g. `\u0041` = "A")|  
 |\v|Vertical tab|0x000B|  
-|\x|Unicode escape sequence similar to "\u" except with variable length.|\x0041 or \x41 = "A"|  
+|\x|Unicode escape sequence similar to "\u" except with variable length.|`\x0041` or `\x41` = "A"|  
+
+> [!WARNING]
+>  When using the `\x` escape sequence and specifying less than 4 hex digits, if the characters that immediately follow the escape sequence are valid hex digits (i.e. 0-9, A-F, and a-f), they will be interpreted as being part of the escape sequence. For example, `\xA1` produces "&#161;", which is code point U+00A1. However, if the next character is "A" or "a", then the escape sequence will instead be interpreted as being `\xA1A` and produce "&#x0A1A;", which is code point U+0A1A. In such cases, specifying all 4 hex digits (e.g. `\x00A1` ) will prevent any possible misinterpretation.  
 
 > [!NOTE]
->  At compile time, verbatim strings are converted to ordinary strings with all the same escape sequences. Therefore, if you view a verbatim string in the debugger watch window, you will see the escape characters that were added by the compiler, not the verbatim version from your source code. For example, the verbatim string @"C:\files.txt" will appear in the watch window as "C:\\\files.txt".  
+>  At compile time, verbatim strings are converted to ordinary strings with all the same escape sequences. Therefore, if you view a verbatim string in the debugger watch window, you will see the escape characters that were added by the compiler, not the verbatim version from your source code. For example, the verbatim string `@"C:\files.txt"` will appear in the watch window as "C:\\\files.txt".  
 
 ## Format Strings  
  A format string is a string whose contents are determined dynamically at runtime. Format strings are created by embedding *interpolated expressions* or placeholders inside of braces within a string. Everything inside the braces (`{...}`) will be resolved to a value and output as a formatted string at runtime. There are two methods to create format strings: string interpolation and composite formatting.