BaseTools/Capsule: Update help for --fw-version and --lsv

Update help to state that --fw-version and -=-lsv are required
for encode operations that sign a payload.

https://bugzilla.tianocore.org/show_bug.cgi?id=1029

Cc: Sean Brogan <sean.brogan@microsoft.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Yonghong Zhu <yonghong.zhu@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Yonghong Zhu <yonghong.zhu@intel.com>