PerformancePkg: Update comments on TscTimerLib

TscTimerLib is a sample implementation that depends on chipset ACPI timer.

(Sync patch r16457 from main trunk.)

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Liming Gao <liming.gao@intel.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>

git-svn-id: https://svn.code.sf.net/p/edk2/code/branches/UDK2014.SP1@16466 6f19259b-4bc3-4df7-8a09-765794883524