Minor doc fix related to ActiveModel::SecurePassword [ci skip]

Author: aditya-kapoor

guides/source/active_model_basics.md

@@ -416,7 +416,6 @@ the Active Model API.
     ```ruby
     class Person
       include ActiveModel::Model
-
     end
     ```
 
@@ -467,7 +466,7 @@ In order to make this work, the model must have an accessor named `password_dige
 The `has_secure_password` will add the following validations on the `password` accessor:
 
 1. Password should be present.
-2. Password should be equal to its confirmation.
+2. Password should be equal to its confirmation (provided +password_confirmation+ is passed along).
 3. The maximum length of a password is 72 (required by `bcrypt` on which ActiveModel::SecurePassword depends)
 
 #### Examples
@@ -493,6 +492,10 @@ person.valid? # => false
 person.password = person.password_confirmation = 'a' * 100
 person.valid? # => false
 
+# When only password is supplied with no password_confirmation.
+person.password = 'aditya'
+person.valid? # => true
+
 # When all validations are passed.
 person.password = person.password_confirmation = 'aditya'
 person.valid? # => true