tox: Fix broken releasenotes target

... due to lack of doc requirements.

Also remove the note about old pip's behavior.

Change-Id: I9b032c854b130109f064e2bc7cfa05d9132f76e6
diff --git a/tox.ini b/tox.ini
index 558e6f8..d5035ba 100644
--- a/tox.ini
+++ b/tox.ini
@@ -35,6 +35,7 @@
 setenv = PYTHONHASHSEED=0
 
 [testenv:releasenotes]
+deps = {[testenv:docs]deps}
 commands =
   sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html