Setting custom release date is not available to users (#163)

* Setting custom release date is not available to users

Based on the forum comment linked below, the documentation was a bit misleading.

https://community.platformio.org/t/is-it-limited-to-6-month-that-the-life-of-old-library-on-platformio-system/17319/29?u=pfeerick

* Removed left over full stop from change of mind change

Author: pfeerick

core/userguide/package/cmd_publish.rst

@@ -65,6 +65,9 @@ Custom release date and time in the next format (UTC): 2014-06-13 17:08:52
 .. option::
     --private
 
+.. note::
+    The permission to set custom release dates and times is only available to Super Admins. 
+
 Restrict access to a package (will not be available publicly). The default is to publish
 a package publicly.