[Docs][CodeGenerator] AsmStreamer doesn't need to be registered anymore

[thegameg]

Summary

• docs/CodeGenerator.rst: From r232423: "There is only one Asm streamer, there is no need for targets to register it.
  
  Instead, have the targets register a TargetStreamer to be use with the asm streamer (if any)."

docs/CodeGenerator.rst

 streamer implement the assembler logic for it.
 
 To make llvm use these classes, the target initialization must call
-TargetRegistry::RegisterAsmStreamer and TargetRegistry::RegisterMCObjectStreamer
-passing callbacks that allocate the corresponding target streamer and pass it
-to createAsmStreamer or to the appropriate object streamer constructor.
+TargetRegistry::RegisterAsmTargetStreamer and
+TargetRegistry::RegisterObjectTargetStreamer passing callbacks that allocate
+the corresponding target streamer and pass it to createAsmStreamer or to the
+appropriate object streamer constructor.
 
 The ``MCContext`` class
 -----------------------