`acquired' was an old boolean variable, but nowadays it is a
rb_thread_t pointer;  "gvl.owner" seems like a more appropriate
name.  And document the contended path including waitq, timer,
and timer_err.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@64581 b2dd03c8-39d4-4d8f-98ff-823fe69b080e