[DOC] Include pathname_builtin.rb documents

.document

@@ -22,6 +22,7 @@ marshal.rb
 numeric.rb
 nilclass.rb
 pack.rb
+pathname_builtin.rb
 ractor.rb
 string.rb
 symbol.rb

ext/.document

@@ -76,8 +76,6 @@ openssl/ossl_x509name.c
 openssl/ossl_x509req.c
 openssl/ossl_x509revoked.c
 openssl/ossl_x509store.c
-pathname/lib
-pathname/pathname.c
 psych/lib
 psych/psych.c
 psych/psych_emitter.c