document OpenSSL::SSL::SSLContext#setup as MT-unsafe

On a cursory inspection, using rb_block_call for extra_chain_cert
is thread-unsafe.  There may be other instances of thread-unsafe
behavior in this method, but one is enough.

* ext/openssl/ossl_ssl.c (ossl_sslctx_setup): document as MT-unsafe
  [ruby-core:73803] [Bug #12069]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@54097 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ChangeLog

@@ -1,3 +1,8 @@
+Mon Mar 14 10:02:23 2016  Eric Wong  <e@80x24.org>
+
+	* ext/openssl/ossl_ssl.c (ossl_sslctx_setup): document as MT-unsafe
+	  [ruby-core:73803] [Bug #12069]
+
 Sun Mar 13 09:43:23 2016  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* include/ruby/win32.h (O_SHARE_DELETE): change to fit Fixnum

ext/openssl/ossl_ssl.c

@@ -687,8 +687,8 @@ ossl_sslctx_set_options(VALUE self, VALUE options)
  *    ctx.setup => nil # thereafter
  *
  * This method is called automatically when a new SSLSocket is created.
- * Normally you do not need to call this method (unless you are writing an
- * extension in C).
+ * However, it is not thread-safe and must be called before creating
+ * SSLSocket objects in a multi-threaded program.
  */
 static VALUE
 ossl_sslctx_setup(VALUE self)