docs, AR already auto-detects primary keys. Closes #13946. [ci skip]

This behavior was introduced since Rails 3.1 (207f266cca)
but the docs were still out of date.

activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb

@@ -120,9 +120,9 @@ module ActiveRecord
       #   The name of the primary key, if one is to be added automatically.
       #   Defaults to +id+. If <tt>:id</tt> is false this option is ignored.
       #
-      #   Also note that this just sets the primary key in the table. You additionally
-      #   need to configure the primary key in the model via +self.primary_key=+.
-      #   Models do NOT auto-detect the primary key from their table definition.
+      #   Note that Active Record models will automatically detect their
+      #   primary key. This can be avoided by using +self.primary_key=+ on the model
+      #   to define the key explicitly.
       #
       # [<tt>:options</tt>]
       #   Any extra options you want appended to the table definition.

activerecord/test/cases/primary_keys_test.rb

@@ -180,6 +180,11 @@ class PrimaryKeysTest < ActiveRecord::TestCase
       assert !col1.equal?(col2)
     end
   end
+
+  def test_auto_detect_primary_key_from_schema
+    MixedCaseMonkey.reset_primary_key
+    assert_equal "monkeyID", MixedCaseMonkey.primary_key
+  end
 end
 
 class PrimaryKeyWithNoConnectionTest < ActiveRecord::TestCase
@@ -214,4 +219,3 @@ if current_adapter?(:MysqlAdapter, :Mysql2Adapter)
     end
   end
 end
-

activerecord/test/models/mixed_case_monkey.rb

@@ -1,5 +1,3 @@
 class MixedCaseMonkey < ActiveRecord::Base
-  self.primary_key = 'monkeyID'
-
   belongs_to :man
 end