restores example depicting the use of true or false in regular font

There was an example here that mentioned reload_plugins?
but it was deleted. This commit puts another example back.

guides/source/api_documentation_guidelines.textile

@@ -133,6 +133,20 @@ h4. Regular Font
 
 When "true" and "false" are English words rather than Ruby keywords use a regular font:
 
+<ruby>
+# Runs all the validations within the specified context. Returns true if no errors are found,
+# false otherwise.
+#
+# If the argument is false (default is +nil+), the context is set to <tt>:create</tt> if
+# <tt>new_record?</tt> is true, and to <tt>:update</tt> if it is not.
+#
+# Validations with no <tt>:on</tt> option will run no matter the context. Validations with
+# some <tt>:on</tt> option will only run in the specified context.
+def valid?(context = nil)
+  ...
+end
+</ruby>
+
 h3. Description Lists
 
 In lists of options, parameters, etc. use a hyphen between the item and its description (reads better than a colon because normally options are symbols):