man/ruby.1: document stack size env variables

[Feature #10197]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47713 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2014-09-26

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47714 b2dd03c8-39d4-4d8f-98ff-823fe69b080e