git-now: update options/caveats

Move the long path name out of the option description and add it to the
caveats instead.

Formula/git-now.rb

@@ -9,9 +9,7 @@ class GitNow < Formula
   depends_on 'gnu-getopt'
 
   def options
-    [
+    [['--zsh-completion', "Install zsh completions."]]
-      ['--zsh-completion', "copy zsh completion function file to #{share}/zsh/functions"]
-    ]
   end
 
   def patches
@@ -26,6 +24,12 @@ class GitNow < Formula
     end
   end
 
+  def caveats; <<-EOS
+    If `--zsh-completion` is specified, zsh completions are installed to:
+      #{share}/zsh/functions
+    EOS
+  end
+
 end
 
 # This patch makes sure GNUtools are used on OSX.