thread.c: fix rdoc
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@36302 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
thread.c: fix rdoc
in "Safe C API". ruby_xmalloc(), ruby_xrealloc() never try acquire
GVL automatically unless GC runs, but had race condtions without GVL
acquired until r36284.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@36302 b2dd03c8-39d4-4d8f-98ff-823fe69b080e