Change the enumerate() example to bind the proxy by reference

Summary:
When compiling without optimizations binding the proxy by
reference is slightly faster (no differences in opt mode), so change
the documentation to recommend this syntax.

The proxy can still be bound by `auto`, `const auto`, and `const
auto&`, in all case behaving as expected and with no overhead in opt
mode. Added a test to make sure these all work.

Reviewed By: yfeldblum, luciang

Differential Revision: D6688958

fbshipit-source-id: 7c6b460a01708786bda7614546fa2e1667f27299