Update rustc/syntax docs now that rustdoc lexes all non-`notrust` code blocks.

This includes blocks made by indentation, so they need to be changed to
explicitly have ```notrust ... ``` fences..
auto merge of #12510 : huonw/rust/fix-compiler-docs, r=alexcrichton

This includes blocks made by indentation, so they need to be changed to
explicitly have ```notrust ... ``` fences..