NetworkPkg: Define the prompt and help information for new PCD.

Define the prompt and help information for PcdMaxIScsiAttemptNumber.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Zhang Lubo <lubo.zhang@intel.com>
Cc: Ye Ting <ting.ye@intel.com>
Cc: Fu Siyuan <siyuan.fu@intel.com>
Cc: Wu Jiaxin <jiaxin.wu@intel.com>
Reviewed-by: Wu Jiaxin <jiaxin.wu@intel.com>