Spell out what will happen if a declaration and definition is included
in the same file, should help people who run into duplicate symbol
errors and search for relevant keywords.

This edit is based on opening issue #11263 erroneously.

Change-Id: I0645a9433b8668d2ede9b9a3f6550d802c26388b
Reviewed-on: https://go-review.googlesource.com/11247Reviewed-by: Ian Lance Taylor <iant@golang.org>