comment-only fix to tools/update-doxygen.sh

tools/update-doxygen.sh

 #!/bin/bash
 #
 # Runs doxygen and stores its results in the skia-autogen repo, so that they
 # can be browsed at http://skia-autogen.googlecode.com/svn/docs/html/index.html
 #
 # The DOXYGEN_TEMPDIR env variable is the working directory within which we will
 # check out the code, generate documentation, and store the doxygen log
 # (by default, /tmp/skia-doxygen). The DOXYGEN_COMMIT env variable determines
 # whether docs should be commited (true by default).
 #
 # Sample Usage:
 #  export DOXYGEN_TEMPDIR=/tmp/doxygen
 #  export DOXYGEN_COMMIT=false
 #  bash update-doxygen.sh
+#
+# To install doxygen on most Linux systems, you can run:
+# sudo apt-get install doxygen
 
 function check_out_docs {
   svn checkout https://skia-autogen.googlecode.com/svn/docs  # writeable
   ret_code=$?
   if [ $ret_code != 0 ]; then
     # docs directory does not exist, skia-autogen must have been reset.
     # Create a non svn docs directory instead.
     mkdir docs
   fi
 }
@@ skipping 75 matching lines @@
 find . -name '*.png'  -exec svn propset svn:mime-type image/png '{}' \;
 
 # Output files with documentation updates.
 echo -e "\n\nThe following are the documentation updates:"
 echo $MODFILES
 
 if $DOXYGEN_COMMIT ; then
   # Commit the updated docs to the subversion repo.
   svn commit --message 'commit doxygen-generated documentation'
 fi