merge revision(s) 57205:

    cont.c: change fiber stack size doc [ci skip]

    It's many years since we had tiny 4KB fiber stacks :<
    point to the manpage instead for current, larger sizes.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_4@57845 b2dd03c8-39d4-4d8f-98ff-823fe69b080e