Updated TreeShaker's usage message.

PiperOrigin-RevId: 362369262

Author: tomball

tree_shaker/src/main/resources/com/google/devtools/treeshaker/TreeShaker.properties

@@ -23,8 +23,10 @@ where possible options include:\n\
   -classpath <path>            Specify where to find user class files.\n\
   --tree-shaker-roots          Specify a file that lists the public root classes and methods.\n\
   -s, --sourcefilelist <file>  Specify a file that lists the source files to be analyzed.\n\
-  -encoding <encoding>         Specify character encoding used by source files\n\
-  -Xbootclasspath:<path>       Boot path used to compile the input sources. (not the tool itself)\n\
+  -o, --output-file <file>     Write output to file.
+  -encoding <encoding>         Specify character encoding used by source files.\n\
+  -source <release>            Provide source compatibility with specified release.
+  -Xbootclasspath:<path>       Boot path used to compile the input sources (not the tool itself).\n\
   -version                     Version information\n\
   -Werror                      Treats all warning as errors.\n\
   -h, --help                   Print this message.