OvmfPkg/MemEncryptSevLib: clean up SetMemoryEncDec() comment block

OvmfPkg/MemEncryptSevLib: clean up SetMemoryEncDec() comment block

Document the "Cr3BaseAddress" parameter, and correct several parameter
references.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Brijesh Singh <brijesh.singh@amd.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Tested-by: Brijesh Singh <brijesh.singh@amd.com>
Reviewed-by: Brijesh Singh <brijesh.singh@amd.com>