Document new function

Author: jansvoboda11

clang/include/clang/Lex/ModuleMap.h

@@ -544,6 +544,9 @@ class ModuleMap {
   /// \returns The found or newly-created module.
   Module *findOrCreateModule(StringRef Name, Module *Parent, bool IsFramework,
                              bool IsExplicit);
+  /// Create new submodule, assuming it does not exist. This function can only
+  /// be called when it is guaranteed that this submodule does not exist yet.
+  /// The parameters have same semantics as \c ModuleMap::findOrCreateModule.
   Module *createModule(StringRef Name, Module *Parent, bool IsFramework,
                        bool IsExplicit);