ShellPkg/ShellPkg.dec: Change comments for PcdShellLibAutoInitialize

When Dynamic command drivers links to ShellLib, the ShellLib
constructor shouldn't be called because the Shell and ShellParameters
protocols don't exist when the driver starts.
So it's required to set PcdShellLibAutoInitialize to FALSE for
dynamic command drivers.
Update the comments in DEC file to describe such requirement
for this PCD.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>