[IR] Add doc about Function::createWithDefaultAttr. NFC

Author: MaskRay

llvm/include/llvm/IR/Function.h

@@ -153,6 +153,11 @@ class Function : public GlobalObject, public ilist_node<Function> {
   static Function *Create(FunctionType *Ty, LinkageTypes Linkage,
                           const Twine &N, Module &M);
 
+  /// Creates a function with some attributes recorded in llvm.module.flags
+  /// applied.
+  ///
+  /// Use this when synthesizing new functions that need attributes that would
+  /// have been set by command line options.
   static Function *createWithDefaultAttr(FunctionType *Ty, LinkageTypes Linkage,
                                          unsigned AddrSpace,
                                          const Twine &N = "",