Document about observer at Ruby 3.4

doc/maintainers.md

@@ -181,11 +181,6 @@ have commit right, others don't.
 * https://github.com/ruby/net-protocol
 * https://rubygems.org/gems/net-protocol
 
-#### lib/observer.rb
-* *unmaintained*
-* https://github.com/ruby/observer
-* https://rubygems.org/gems/observer
-
 #### lib/open3.rb
 * *unmaintained*
 * https://github.com/ruby/open3
@@ -481,6 +476,9 @@ have commit right, others don't.
 #### bigdecimal
 * https://github.com/ruby/bigdecimal
 
+#### observer
+* https://github.com/ruby/observer
+
 ## Platform Maintainers
 ### mswin64 (Microsoft Windows)
 * NAKAMURA Usaku (usa)

doc/standard_library.rdoc

@@ -52,7 +52,6 @@ IRB:: Interactive Ruby command-line tool for REPL (Read Eval Print Loop)
 OptionParser:: Ruby-oriented class for command-line option analysis
 Logger:: Provides a simple logging utility for outputting messages
 Net::HTTP:: HTTP client api for Ruby
-Observable:: Provides a mechanism for publish/subscribe pattern in Ruby
 Open3:: Provides access to stdin, stdout and stderr when running other programs
 OpenStruct:: Class to build custom data structures, similar to a Hash
 OpenURI:: An easy-to-use wrapper for Net::HTTP, Net::HTTPS and Net::FTP
@@ -130,3 +129,4 @@ Mutex_m:: Mixin to extend objects to be handled like a Mutex
 GetoptLong:: Parse command line options similar to the GNU C getopt_long()
 Base64:: Support for encoding and decoding binary data using a Base64 representation
 BigDecimal:: Provides arbitrary-precision floating point decimal arithmetic
+Observable:: Provides a mechanism for publish/subscribe pattern in Ruby