Ensure AR::Core#{slice,values_at} appear in RDoc [ci-skip]

When a `:method:` doc is immediately followed by the `private` keyword,
RDoc will hide that doc as if it were a private method.

In rails#48324, the `:method:` docs for `ActiveRecord::Core#slice` and
`ActiveRecord::Core#values_at` were swapped in an attempt to force
`values_at` to appear in the rendered docs.  However, doing so causes
`slice` to dissappear from the rendered docs.

To ensure that `slice` and `values_at` both appear in the rendered docs,
this commit moves their `:method:` docs away from the `private` keyword
in `ActiveRecord::Core`.

Author: jonathanhefner

activerecord/lib/active_record/core.rb

@@ -536,6 +536,27 @@ def encode_with(coder)
       coder["active_record_yaml_version"] = 2
     end
 
+    ##
+    # :method: slice
+    #
+    # :call-seq: slice(*methods)
+    #
+    # Returns a hash of the given methods with their names as keys and returned
+    # values as values.
+    #
+    #--
+    # Implemented by ActiveModel::Access#slice.
+
+    ##
+    # :method: values_at
+    #
+    # :call-seq: values_at(*methods)
+    #
+    # Returns an array of the values returned by the given methods.
+    #
+    #--
+    # Implemented by ActiveModel::Access#values_at.
+
     # Returns true if +comparison_object+ is the same exact object, or +comparison_object+
     # is of the same type and +self+ has an ID and it is equal to +comparison_object.id+.
     #
@@ -701,27 +722,6 @@ def pretty_print(pp)
       end
     end
 
-    ##
-    # :method: values_at
-    #
-    # :call-seq: values_at(*methods)
-    #
-    # Returns an array of the values returned by the given methods.
-    #
-    #--
-    # Implemented by ActiveModel::Access#values_at.
-
-    ##
-    # :method: slice
-    #
-    # :call-seq: slice(*methods)
-    #
-    # Returns a hash of the given methods with their names as keys and returned
-    # values as values.
-    #
-    #--
-    # Implemented by ActiveModel::Access#slice.
-
     private
       # +Array#flatten+ will call +#to_ary+ (recursively) on each of the elements of
       # the array, and then rescues from the possible +NoMethodError+. If those elements are