explains Array.wrap directly, rather by comparison with Kernel#Array which is too obscure, leaves the comparison to document the differences, and adds a comparison with the related idiom that uses the splat operator

Author: fxn

activesupport/lib/active_support/core_ext/array/wrap.rb

@@ -1,5 +1,17 @@
 class Array
-  # <tt>Array.wrap</tt> is like <tt>Kernel#Array</tt> except:
+  # Wraps its argument in an array unless it is already an array (or array-like).
+  #
+  # Specifically:
+  #
+  # * If the argument is +nil+ an empty list is returned.
+  # * Otherwise, if the argument responds to +to_ary+ it is invoked, and its result returned.
+  # * Otherwise, returns an array with the argument as its single element.
+  #
+  #   Array.wrap(nil)       # => []
+  #   Array.wrap([1, 2, 3]) # => [1, 2, 3]
+  #   Array.wrap(0)         # => [0]
+  # 
+  # This method is similar in purpose to <tt>Kernel#Array</tt>, but there are some differences:
   #
   # * If the argument responds to +to_ary+ the method is invoked. <tt>Kernel#Array</tt>
   # moves on to try +to_a+ if the returned value is +nil+, but <tt>Arraw.wrap</tt> returns
@@ -15,6 +27,15 @@ class Array
   #
   #   Array("foo\nbar")        # => ["foo\n", "bar"], in Ruby 1.8
   #   Array.wrap("foo\nbar")   # => ["foo\nbar"]
+  #
+  # There's also a related idiom that uses the splat operator:
+  #
+  #   [*object]
+  #
+  # which returns <tt>[nil]</tt> for +nil+, and calls to <tt>Array(object)</tt> otherwise.
+  #
+  # Thus, in this case the behavior is different for +nil+, and the differences with
+  # <tt>Kernel#Array</tt> explained above apply to the rest of +object+s.
   def self.wrap(object)
     if object.nil?
       []

railties/guides/source/active_support_core_extensions.textile

@@ -2214,14 +2214,27 @@ NOTE: Defined in +active_support/core_ext/array/conversions.rb+.
 
 h4. Wrapping
 
-The class method +Array.wrap+ behaves like the function +Array()+ except:
+The method +Array.wrap+ wraps its argument in an array unless it is already an array (or array-like).
+
+Specifically:
+
+* If the argument is +nil+ an empty list is returned.
+* Otherwise, if the argument responds to +to_ary+ it is invoked, and its result returned.
+* Otherwise, returns an array with the argument as its single element.
+
+<ruby>
+Array.wrap(nil)       # => []
+Array.wrap([1, 2, 3]) # => [1, 2, 3]
+Array.wrap(0)         # => [0]
+</ruby>
+
+This method is similar in purpose to <tt>Kernel#Array</tt>, but there are some differences:
 
 * If the argument responds to +to_ary+ the method is invoked. <tt>Kernel#Array</tt> moves on to try +to_a+ if the returned value is +nil+, but <tt>Arraw.wrap</tt> returns such a +nil+ right away.
 * If the returned value from +to_ary+ is neither +nil+ nor an +Array+ object, <tt>Kernel#Array</tt> raises an exception, while <tt>Array.wrap</tt> does not, it just returns the value.
 * It does not call +to_a+ on the argument, though special-cases +nil+ to return an empty array.
 
-
- that it does not try to call +to_a+ on its argument. That changes the behavior for enumerables:
+The last point is particularly worth comparing for some enumerables:
 
 <ruby>
 Array.wrap(:foo => :bar) # => [{:foo => :bar}]
@@ -2231,6 +2244,16 @@ Array.wrap("foo\nbar")   # => ["foo\nbar"]
 Array("foo\nbar")        # => ["foo\n", "bar"], in Ruby 1.8
 </ruby>
 
+There's also a related idiom that uses the splat operator:
+
+<ruby>
+[*object]
+</ruby>
+
+which returns +[nil]+ for +nil+, and calls to <tt>Array(object)</tt> otherwise
+
+Thus, in this case the behavior is different for +nil+, and the differences with <tt>Kernel#Array</tt> explained above apply to the rest of +object+s.
+
 NOTE: Defined in +active_support/core_ext/array/wrap.rb+.
 
 h4. Grouping