ctags: Remove conflict with emacs and add caveat

Closes Homebrew/homebrew#16841.

Signed-off-by: Misty De Meo <mistydemeo@gmail.com>

Formula/ctags.rb

@@ -7,9 +7,6 @@ class Ctags < Formula
 
   head 'https://ctags.svn.sourceforge.net/svnroot/ctags/trunk'
 
-  conflicts_with 'emacs',
-    :because => 'Both ctags and emacs install a `ctags` executable.'
-
   depends_on :autoconf => :build if build.head?
 
   def patches
@@ -28,4 +25,18 @@ class Ctags < Formula
                           "--with-readlib"
     system "make install"
   end
+
+  def caveats
+    <<-EOS.undent
+      Under some circumstances, emacs and ctags can conflict. By default,
+      emacs provides an executable `ctags` that would conflict with the
+      executable of the same name that ctags provides. To prevent this,
+      Homebrew removes the emacs `ctags` and its manpage before linking.
+
+      However, if you install emacs with the `--keep-ctags` option, then
+      the `ctags` emacs provides will not be removed. In that case, you
+      won't be able to install ctags successfully. It will build but not
+      link.
+    EOS
+  end
 end