content: add explanation of slice expression syntax
content: add explanation of slice expression syntax

Fixes golang/tour#66
Fixes golang/tour#67
Fixes golang/tour#68
Fixes golang/tour#177
Fixes golang/tour#209

Change-Id: Ie3d944a5baacd471a386dfd849f55b41dbd5db95
Reviewed-on: https://go-review.googlesource.com/44930
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

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