[DOC] Stop using `:enddoc:` in rdoc files

In rdoc files, all documents belong to the top level of each file, and
`:enddoc:` turns off `#document_self` for the entire document in the
current file.  This disables cross-references to labels in that file.
Instead use `:stopdoc:` so that parsing can re-start by `:startdoc:`
after the local variables.

doc/extension.ja.rdoc

@@ -1863,7 +1863,7 @@ Ruby 3.0 から、Ruby プログラムを並列に実行するための仕組み
 Ractor をサポートするための詳細は、extension.rdoc の「Appendix F. Ractor
 support」を参照してください。
 
-
-:enddoc: Local variables:
-:enddoc: fill-column: 60
-:enddoc: end:
+:stopdoc: Local variables:
+:stopdoc: fill-column: 60
+:stopdoc: end:
+:startdoc:

doc/extension.rdoc

@@ -2274,6 +2274,7 @@ There are possibly other points or requirements which must be considered in the
 making of a Ractor-safe extension.  This document will be extended as they are
 discovered.
 
-:enddoc: Local variables:
-:enddoc: fill-column: 70
-:enddoc: end:
+:stopdoc: Local variables:
+:stopdoc: fill-column: 70
+:stopdoc: end:
+:startdoc: