add insight about parallel test-all

see also https://bugs.ruby-lang.org/projects/ruby/wiki/DeveloperHowto

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@53049 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

common.mk

@@ -1085,7 +1085,7 @@ help: PHONY
 	"  gdb-ruby:        runs test.rb by ruby under gdb" \
 	"  check:           equals make test test-all" \
 	"  test:            ruby core tests" \
-	"  test-all:        all ruby tests [TESTS=<test files>]" \
+	"  test-all:        all ruby tests [TESTS=\"-j4 <test files>\"]" \
 	"  test-rubyspec:   run RubySpec test suite" \
 	"  update-rubyspec: update local copy of RubySpec" \
 	"  benchmark:       benchmark this ruby and COMPARE_RUBY." \