Update usage disclaimer on FieldOptions.ctype

FieldOptions.ctype is public and CORD support is implemented for [ctype=CORD] on "bytes" fields. Include a public comment for CORD about use and supported types.

PiperOrigin-RevId: 524383969

src/google/protobuf/descriptor.proto

@@ -569,13 +569,21 @@ message MessageOptions {
 message FieldOptions {
   // The ctype option instructs the C++ code generator to use a different
   // representation of the field than it normally would.  See the specific
-  // options below.  This option is not yet implemented in the open source
-  // release -- sorry, we'll try to include it in a future version!
+  // options below.  This option is only implemented to support use of
+  // [ctype=CORD] and [ctype=STRING] (the default) on non-repeated fields of
+  // type "bytes" in the open source release -- sorry, we'll try to include
+  // other types in a future version!
   optional CType ctype = 1 [default = STRING];
   enum CType {
     // Default mode.
     STRING = 0;
 
+    // The option [ctype=CORD] may be applied to a non-repeated field of type
+    // "bytes". It indicates that in C++, the data should be stored in a Cord
+    // instead of a string.  For very large strings, this may reduce memory
+    // fragmentation. It may also allow better performance when parsing from a
+    // Cord, or when parsing with aliasing enabled, as the parsed Cord may then
+    // alias the original buffer.
     CORD = 1;
 
     STRING_PIECE = 2;