Add reasons for pdf2image, poppler, and xpdf conflicts

Formula/pdf2image.rb

@@ -7,8 +7,8 @@ class Pdf2image < Formula
 
   depends_on :x11
 
-  conflicts_with 'poppler'
-  conflicts_with 'xpdf'
+  conflicts_with 'poppler', 'xpdf',
+    :because => 'pdf2image, poppler, and xpdf install conflicting executables'
 
   def install
     system "./configure", "--prefix=#{prefix}"

Formula/poppler.rb

@@ -23,8 +23,8 @@ class Poppler < Formula
   depends_on 'glib' => :optional
   depends_on 'cairo' if build.with? 'glib' # Needs a newer Cairo build than OS X 10.6.7 provides
 
-  conflicts_with 'pdf2image'
-  conflicts_with 'xpdf'
+  conflicts_with 'pdf2image', 'xpdf',
+    :because => 'poppler, pdf2image, and xpdf install conflicting executables'
 
   def install
     if build.with? 'qt4'

Formula/xpdf.rb

@@ -8,8 +8,8 @@ class Xpdf < Formula
   depends_on 'lesstif'
   depends_on :x11
 
-  conflicts_with 'pdf2image'
-  conflicts_with 'poppler'
+  conflicts_with 'pdf2image', 'poppler',
+    :because => 'xpdf, pdf2image, and poppler install conflicting executables'
 
   # see: http://gnats.netbsd.org/45562
   def patches; DATA; end