task: install completion into site-functions

`_task` needs to be installed into `share/zsh/site-functions`
not `share/zsh/functions`.

Files in `share/zsh/functions` must not change unless zsh is
upgraded and zsh is restarted.  `site-functions` can change during a zsh
process's lifetime.

Closes Homebrew/homebrew#11939.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Formula/task.rb

@@ -13,6 +13,15 @@ class Task < Formula
     system "cmake #{std_cmake_parameters}"
     system "make install"
     (etc+'bash_completion.d').install 'scripts/bash/task_completion.sh'
-    (share+'zsh/functions').install   'scripts/zsh/_task'
+    (share+'zsh/site-functions').install   'scripts/zsh/_task'
+  end
+
+  def caveats; <<-EOS.undent
+    Bash completion has been installed to:
+      #{etc}/bash_completion.d
+
+    zsh completion has been installed to:
+      #{HOMEBREW_PREFIX}/share/zsh/site-functions
+    EOS
   end
 end