CPP-6594 S817 Add C++23 context and Unicode string literal examples

rules/S817/cfamily/rule.adoc

@@ -1,26 +1,34 @@
 == Why is this an issue?
 
-Concatenation of wide and narrow string literals has not always been supported in C or {cpp}, and even when supported, the meaning may be unclear to the reader. Concatenation of string literals with different encodings is only conditionally supported, and may be removed in a future version of the language.
-
+Concatenation of wide and narrow string literals has not always been supported in C or {cpp}, and even when supported, the meaning may be unclear to the reader. Furthermore, concatenation of string literals with different encodings was only conditionally supported prior to {cpp}23, and is now explicitly ill-formed as of {cpp}23.
 
 Therefore, only string literals with the same prefix should be concatenated together.
 
 
 === Noncompliant code example
 
-[source,cpp]
+[source,cpp,diff-id=1,diff-type=noncompliant]
 ----
 wchar_t n_array[] = "Hello" L"World";     // Noncompliant
 wchar_t w_array[] = L"Hello" "World";     // Noncompliant
+auto u8_array = "Hello" u8"World";        // Noncompliant
+auto u_array = u"Hello" "World";          // Noncompliant
+
+// Mixed encoding prefixes (ill-formed as of C++23)
+auto mixed1 = L"Hello" u8"World";         // Noncompliant
+auto mixed2 = u"Hello" U"World";          // Noncompliant
+auto mixed3 = u8"Hello" u"World";         // Noncompliant
 ----
 
 
 === Compliant solution
 
-[source,cpp]
+[source,cpp,diff-id=1,diff-type=compliant]
 ----
-char_t n_array[] = "Hello" "World";     // Compliant
-wchar_t w_array[] = L"Hello" L"World";	// Compliant
+char n_array[] = "Hello" "World";         // Compliant
+wchar_t w_array[] = L"Hello" L"World";    // Compliant
+auto u8_array = u8"Hello" u8"World";      // Compliant
+auto u_array = u"Hello" u"World";         // Compliant
 ----