[SP][Spec] Updated SP spec docs.
[SP][Spec] Updated SP spec docs.

This patch mainly fill out the function and arguments intro within
'Methods' section. Now the SP spec content should be completed.

Fix: #312

halton/realsense-extensions-crosswalk,jondong/realsense-extensions-crosswalk,crosswalk-project/realsense-extensions-crosswalk,halton/realsense-extensions-crosswalk,halton/realsense-extensions-crosswalk,jondong/realsense-extensions-crosswalk,crosswalk-project/realsense-extensions-crosswalk,jondong/realsense-extensions-crosswalk,crosswalk-project/realsense-extensions-crosswalk,halton/realsense-extensions-crosswalk,crosswalk-project/realsense-extensions-crosswalk,jondong/realsense-extensions-crosswalk