GC documentation update

* gc.c (GC_HEAP_FREE_SLOTS): move definition to match use order
  (RUBY_GC_HEAP_GROWTH_SLOTS): s/factor/number of slots/

* man/ruby.1: add section for GC environment variables
  [Feature #10197]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48855 b2dd03c8-39d4-4d8f-98ff-823fe69b080e