Merge pull request #2453 from p8/docs/replace-ruby-doc.org

Replace reference with official documentation

Author: soutaro

core/thread.rbs

@@ -829,13 +829,6 @@ class Thread < Object
   #
   def self.each_caller_location: () { (Backtrace::Location) -> void } -> nil
 
-  # Wraps the block in a single, VM-global
-  # [Mutex\#synchronize](https://ruby-doc.org/core-2.6.3/Mutex.html#method-i-synchronize)
-  # , returning the value of the block. A thread executing inside the
-  # exclusive section will only block other threads which also use the
-  # [::exclusive](Thread.downloaded.ruby_doc#method-c-exclusive) mechanism.
-  def self.exclusive: () { () -> untyped } -> untyped
-
   # <!--
   #   rdoc-file=thread.c
   #   - Thread.exit   -> thread