+ * Set a property to be stored in the page_props database table.
+ *
+ * page_props is a key value store indexed by the page ID. This allows
+ * the parser to set a property on a page which can then be quickly
+ * retrieved given the page ID or via a DB join when given the page
+ * title.
+ *
+ * setProperty() is thus used to propagate properties from the parsed
+ * page to request contexts other than a page view of the currently parsed
+ * article.
+ *
+ * Some applications examples:
+ *
+ * * To implement hidden categories, hiding pages from category listings
+ * by storing a property.
+ *
+ * * Overriding the displayed article title.
+ * @see ParserOutput::setDisplayTitle()
+ *
+ * * To implement image tagging, for example displaying an icon on an
+ * image thumbnail to indicate that it is listed for deletion on
+ * Wikimedia Commons.
+ * This is not actually implemented, yet but would be pretty cool.
+ *
+ * @note: Do not use setProperty() to set a property which is only used
+ * in a context where the ParserOutput object itself is already available,
+ * for example a normal page view. There is no need to save such a property
+ * in the database since it the text is already parsed. You can just hook
+ * OutputPageParserOutput and get your data out of the ParserOutput object.
+ *
+ * If you are writing an extension where you want to set a property in the
+ * parser which is used by an OutputPageParserOutput hook, you have to
+ * associate the extension data directly with the ParserOutput object.
+ * Since MediaWiki 1.21, you can use setExtensionData() to do this:
+ *
+ * @par Example:
+ * @code
+ * $parser->getOutput()->setExtensionData( 'my_ext_foo', '...' );
+ * @endcode
+ *
+ * And then later, in OutputPageParserOutput or similar:
+ *
+ * @par Example:
+ * @code
+ * $output->getExtensionData( 'my_ext_foo' );
+ * @endcode
+ *
+ * In MediaWiki 1.20 and older, you have to use a custom member variable
+ * within the ParserOutput object:
+ *
+ * @par Example:
+ * @code
+ * $parser->getOutput()->my_ext_foo = '...';
+ * @endcode
+ *