Document the Message parameter of getCustomDiagID()

A lot of callers have been using this facility incorrectly.

llvm-svn: 197920

clang/include/clang/Basic/Diagnostic.h

@@ -591,6 +591,9 @@ public:
   ///
   /// If this is the first request for this diagnostic, it is registered and
   /// created, otherwise the existing ID is returned.
+  ///
+  /// \param Message A fixed diagnostic format string that will be hashed and
+  /// mapped to a unique DiagID.
   unsigned getCustomDiagID(Level L, StringRef Message) {
     return Diags->getCustomDiagID((DiagnosticIDs::Level)L, Message);
   }

clang/lib/Basic/DiagnosticIDs.cpp

@@ -311,6 +311,9 @@ DiagnosticIDs::~DiagnosticIDs() {
 /// getCustomDiagID - Return an ID for a diagnostic with the specified message
 /// and level.  If this is the first request for this diagnostic, it is
 /// registered and created, otherwise the existing ID is returned.
+///
+/// \param Message A fixed diagnostic format string that will be hashed and
+/// mapped to a unique DiagID.
 unsigned DiagnosticIDs::getCustomDiagID(Level L, StringRef Message) {
   if (CustomDiagInfo == 0)
     CustomDiagInfo = new diag::CustomDiagInfo();