AS guide: documents Class#subclasses

railties/guides/source/active_support_core_extensions.textile

@@ -977,6 +977,39 @@ If for whatever reason an application loads the definition of a mailer class and
 
 NOTE: Defined in +active_support/core_ext/class/delegating_attributes.rb+.
 
+h4. Descendants
+
+h5. +subclasses+
+
+The +subclasses+ method returns the names of all the anonymous or reachable descendants of its receiver as an array of strings:
+
+<ruby>
+class C; end
+C.subclasses # => []
+
+Integer.subclasses # => ["Bignum", "Fixnum"]
+
+module M
+  class A; end
+  class B1 < A; end
+  class B2 < A; end
+end
+
+module N
+  class C < M::B1; end
+end
+
+M::A.subclasses # => ["N::C", "M::B2", "M::B1"]
+</ruby>
+
+The order in which these class names are returned is unspecified.
+
+See also +Object#subclasses_of+ in "Extensions to All Objects FIX THIS LINK":FIXME.
+
+WARNING: This method is redefined in some Rails core classes.
+
+NOTE: Defined in +active_support/core_ext/class/subclasses.rb+.
+
 h3. Extensions to +String+
 
 h4. Output Safety