[IR] Update getOrInsertFunction() docs for opaque pointers (NFC)

This can no longer return a bitcast, but the function type in
FunctionCallee may differ from the function type of the function.

Author: nikic

llvm/include/llvm/IR/Module.h

@@ -385,30 +385,23 @@ class LLVM_EXTERNAL_VISIBILITY Module {
 /// @name Function Accessors
 /// @{
 
-  /// Look up the specified function in the module symbol table. Four
-  /// possibilities:
-  ///   1. If it does not exist, add a prototype for the function and return it.
-  ///   2. Otherwise, if the existing function has the correct prototype, return
-  ///      the existing function.
-  ///   3. Finally, the function exists but has the wrong prototype: return the
-  ///      function with a constantexpr cast to the right prototype.
+  /// Look up the specified function in the module symbol table. If it does not
+  /// exist, add a prototype for the function and return it. Otherwise, return
+  /// the existing function.
   ///
   /// In all cases, the returned value is a FunctionCallee wrapper around the
-  /// 'FunctionType *T' passed in, as well as a 'Value*' either of the Function or
-  /// the bitcast to the function.
+  /// 'FunctionType *T' passed in, as well as the 'Value*' of the Function. The
+  /// function type of the function may differ from the function type stored in
+  /// FunctionCallee if it was previously created with a different type.
   ///
   /// Note: For library calls getOrInsertLibFunc() should be used instead.
   FunctionCallee getOrInsertFunction(StringRef Name, FunctionType *T,
                                      AttributeList AttributeList);
 
   FunctionCallee getOrInsertFunction(StringRef Name, FunctionType *T);
 
-  /// Look up the specified function in the module symbol table. If it does not
-  /// exist, add a prototype for the function and return it. This function
-  /// guarantees to return a constant of pointer to the specified function type
-  /// or a ConstantExpr BitCast of that type if the named function has a
-  /// different type. This version of the method takes a list of
-  /// function arguments, which makes it easier for clients to use.
+  /// Same as above, but takes a list of function arguments, which makes it
+  /// easier for clients to use.
   template <typename... ArgsTy>
   FunctionCallee getOrInsertFunction(StringRef Name,
                                      AttributeList AttributeList, Type *RetTy,