Add documentation for -Wwrite-strings

This is the first instance where we've really needed to add
documentation for a diagnostic group, but -Wwrite-strings really
deserves it.

This warning option changes the semantic behavior of code, so enabling
it can cause code to break (and disabling it can too). That's worth
calling out loudly in our documentation.

clang/include/clang/Basic/Diagnostic.td

@@ -55,11 +55,11 @@ class DiagCategory<string Name> {
 }
 
 // Diagnostic Groups.
-class DiagGroup<string Name, list<DiagGroup> subgroups = []> {
+class DiagGroup<string Name, list<DiagGroup> subgroups = [], code docs = [{}]> {
   string GroupName = Name;
   list<DiagGroup> SubGroups = subgroups;
   string CategoryName = "";
-  code Documentation = [{}];
+  code Documentation = docs;
 }
 class InGroup<DiagGroup G> { DiagGroup Group = G; }
 //class IsGroup<string Name> { DiagGroup Group = DiagGroup<Name>; }

clang/include/clang/Basic/DiagnosticDocs.td

@@ -81,3 +81,9 @@ Diagnostic flags
 }];
 }
 
+defvar GCCWriteStringsDocs = [{
+**Note:** enabling this warning in C will change the semantic behavior of the
+program by treating all string literals as having type ``const char *``
+instead of ``char *``. This can cause unexpected behaviors with type-sensitive
+constructs like ``_Generic``.
+}];

clang/include/clang/Basic/DiagnosticGroups.td

@@ -847,7 +847,8 @@ def WritableStrings : DiagGroup<"writable-strings", [DeprecatedWritableStr]>;
 //
 // FIXME: Should this affect C++11 (where this is an error,
 //        not just deprecated) or not?
-def GCCWriteStrings : DiagGroup<"write-strings" , [WritableStrings]>;
+def GCCWriteStrings : DiagGroup<"write-strings" , [WritableStrings],
+                                GCCWriteStringsDocs>;
 
 def CharSubscript : DiagGroup<"char-subscripts">;
 def LargeByValueCopy : DiagGroup<"large-by-value-copy">;