clarify -F command-line option usage more

lib/rscons/cli.rb

@@ -5,7 +5,7 @@ USAGE = <<EOF
 Usage: #{$0} [global options] [operation] [operation options]
 
 Global options:
-  -F, --show-failure          Show previous failed command(s) and exit
+  -F, --show-failure          Show failed command log from previous build and exit
   --version                   Show rscons version and exit
   -h, --help                  Show rscons help and exit
   -r COLOR, --color=COLOR     Set color mode (off, auto, force)

lib/rscons/environment.rb

@@ -284,7 +284,7 @@ module Rscons
       unless @process_failures.empty?
         msg = @process_failures.join("\n")
         if Cache.instance["failed_commands"].size > 0
-          msg += "\nRun `#{$0} -F` to see the failed command(s)."
+          msg += "\nUse -F to view the failed command log from the previous build operation"
         end
         raise BuildError.new(msg)
       end