content: explain shift operator in constants example
content: explain shift operator in constants example

This addresses concerns raised in
https://go-review.googlesource.com/#/c/19325/

The literal number 1267650600228229401496703205376 is potentially
hard read. 1,267,650,600,228,229,401,496,703,205,376 was suggested
as an alternative.

The == operator has not yet been used in an example in the tour,
and so using it could potentially also confuse people.

Additionally it seems useful to hint at what << and >> mean, rather
than glossing over it entirely.

Change-Id: I89652eea138a91f3da5b7629b0edfaa96dcdbc23
Reviewed-on: https://go-review.googlesource.com/19370
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>
Reviewed-by: Ralph Corderoy <d6424ecc823ca663bdcbaea8e2d9df58a4d0e2b8@inputplus.co.uk>

zorion/go-tour-ca,shuLhan/go-tour-id,scryner/tour,dupoxy/go-tour-fr,Go-zh/tour,shuLhan/go-tour-id,golang/tour,scryner/tour,golians/tour,shuLhan/tour,zorion/go-tour-ca,golians/tour,shuLhan/go-tour-id,golians/tour,dupoxy/go-tour-fr,Go-zh/tour,Go-zh/tour,shuLhan/tour,Go-zh/tour,dupoxy/go-tour-fr,scryner/tour,scryner/tour,shuLhan/tour