Explicitly state that master won't be there by default

lib/cocoapods/command/update.rb

@@ -22,7 +22,7 @@ module Pod
       def self.options
         [
           ['--sources=https://github.com/artsy/Specs,master', 'The sources from which to update dependent pods. ' \
-           'Multiple sources must be comma-delimited.'],
+           'Multiple sources must be comma-delimited. The master repo will not be included by default with this option.'],
         ].concat(super)
       end