Add doc for `:touch` option of AR::Base#save

ActiveRecord::Base `save` and `save!` take an option boolean
`:touch` parameter since #18225 (stems from #18202).

This commit document that parameter.

[ci skip]

activerecord/lib/active_record/persistence.rb

@@ -109,6 +109,10 @@ def persisted?
     # validate: false, validations are bypassed altogether. See
     # ActiveRecord::Validations for more information.
     #
+    # By default, #save also sets the +updated_at+/+updated_on+ attributes to
+    # the current time. However, if you supply <tt>touch: false</tt>, these
+    # timestamps will not be updated.
+    #
     # There's a series of callbacks associated with +save+. If any of the
     # <tt>before_*</tt> callbacks return +false+ the action is cancelled and
     # +save+ returns +false+. See ActiveRecord::Callbacks for further
@@ -131,6 +135,10 @@ def save(*args)
     # ActiveRecord::RecordInvalid gets raised. See ActiveRecord::Validations
     # for more information.
     #
+    # By default, #save! also sets the +updated_at+/+updated_on+ attributes to
+    # the current time. However, if you supply <tt>touch: false</tt>, these
+    # timestamps will not be updated.
+    #
     # There's a series of callbacks associated with <tt>save!</tt>. If any of
     # the <tt>before_*</tt> callbacks return +false+ the action is cancelled
     # and <tt>save!</tt> raises ActiveRecord::RecordNotSaved. See