Reword a sentence bringing confusion about docker links

As [discovered][1] doing user support, the sentence `You've learned that a link creates a
source container that can provide information about itself to a recipient container` brings
confusion.

[1]: http://stackoverflow.com/questions/26652877/how-to-avoid-redundant-container-linking-in-docker-when-propagating-ip-addresses/26654203?noredirect=1#comment41945048_26654203

Signed-off-by: Thomas LEVEIL <thomasleveil@gmail.com>
Merge pull request #8884 from thomasleveil/userguide-so26652877

Reword a sentence bringing confusion about docker links