Upgrader: Make clearing the Plugin and Theme update caches optional during install and upgrade proceedures. See #22704


git-svn-id: https://develop.svn.wordpress.org/trunk@25272 602fd350-edb4-49c9-b593-d223f7449a82
Short descriptions for inline docs should end with a period, per the vast majority of core. see #25229.


git-svn-id: https://develop.svn.wordpress.org/trunk@25273 602fd350-edb4-49c9-b593-d223f7449a82