Add Unicode escape sequence scenario to CS1012 error documentation (#47962)

* Initial plan

* Add Unicode escape sequence scenario to CS1012 error documentation

Co-authored-by: BillWagner <[email protected]>

* Update docs/csharp/misc/cs1012.md

* Update docs/csharp/misc/cs1012.md

* Update docs/csharp/misc/cs1012.md

* Update docs/csharp/misc/cs1012.md

Co-authored-by: Genevieve Warren <[email protected]>

---------

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: BillWagner <[email protected]>
Co-authored-by: Bill Wagner <[email protected]>
Co-authored-by: Genevieve Warren <[email protected]>

Author: 3 people

docs/csharp/misc/cs1012.md

@@ -12,9 +12,7 @@ ms.assetid: 4acc5fe0-da47-4882-b7d8-557767d7cf03
 
 Too many characters in character literal  
 
- An attempt was made to initialize a [char](../language-reference/builtin-types/char.md) constant with more than one character.  
-  
- CS1012 can also occur when doing data binding. For example the following line will give an error:  
+ An attempt was made to initialize a [char](../language-reference/builtin-types/char.md) constant with more than one character. CS1012 can occur when doing data binding. CS1012 can also occur when using Unicode escape sequences that exceed the UTF-16 range. A character literal represents a single character as a UTF-16 code unit, so Unicode values greater than 0xFFFF (65535) aren't valid in character literals. For example, the following code gives an error:  
 
  `<%# DataBinder.Eval(Container.DataItem, 'doctitle') %>`  
 
@@ -30,9 +28,12 @@ class Sample
 {  
    static void Main()  
    {  
-      char a = 'xx';   // CS1012  
-      char a2 = 'x';   // OK  
-      System.Console.WriteLine(a2);  
+      char a = 'xx';   // CS1012 - too many characters
+      char b = '\U00010000';   // CS1012 - exceeds UTF-16 range (> 0xFFFF)
+      
+      char c = 'x';    // OK  
+      char d = '\u0061';   // OK - represents 'a' within UTF-16 range
+      System.Console.WriteLine($"{c}, {d}");  
    }  
 }  
 ```