Unify `cattr_*` interface: allow to pass a block to `cattr_reader`.

Example:

    class A
      cattr_reader(:defr) { 'default_reader_value' }
    end
    A.defr # => 'default_reader_value'

1 files changed, 9 insertions, 0 deletions

diff --git a/guides/source/active_support_core_extensions.md b/guides/source/active_support_core_extensions.md
index 648036fb3f..84a169b3b9 100644
--- a/guides/source/active_support_core_extensions.md
+++ b/guides/source/active_support_core_extensions.md
@@ -1093,6 +1093,15 @@ end
 
 we can access `field_error_proc` in views.
 
+Also, you can pass a block to `cattr_*` to set up the attribute with a default value:
+
+```ruby
+class MysqlAdapter < AbstractAdapter
+  # Generates class methods to access @@emulate_booleans with default value of true.
+  cattr_accessor(:emulate_booleans) { true }
+end
+```
+
 The generation of the reader instance method can be prevented by setting `:instance_reader` to `false` and the generation of the writer instance method can be prevented by setting `:instance_writer` to `false`. Generation of both methods can be prevented by setting `:instance_accessor` to `false`. In all cases, the value must be exactly `false` and not any false value.
 
 ```ruby