[DOC] Improve TCEMAIN.clearCacheCmd example

Since TYPO3CMS 6.0 it is possible to flush page and pagesection cache by given tag.

Change-Id: I2aba5625d09a4af9aa1a2a9d38ab0a452d0cf0f5
Reviewed-on: http://review.typo3.org/34465
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>