ShellPkg: Update the help information for 'setvar' command to follow Shell behavior.

ShellPkg: Update the help information for 'setvar' command to follow Shell behavior.

Since Shell will remove the quotes in parameters, setvar cannot receive the quotes from ="ascii" or =L"unicode". User should add ^ to escape quotes in setvar data.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Qiu Shumin <shumin.qiu@intel.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18454 6f19259b-4bc3-4df7-8a09-765794883524