content: explain the example in the channels slide

There was a suggestion to simplify the example in order to
get rid of the somewhat complicated-looking code that split
the slice in half:


I think that the example is actually simple enough, and that
giving the reader a bit of context up front about what the code
is accomplishing will make it seem simpler without actually
changing the code.

Fixes golang/go#13894

Change-Id: I1dfe8ec6de07bb1d74713d824e93c34835139933
Reviewed-by: Andrew Gerrand <>
1 file changed