Add comments for nav_msgs defintions

Signed-off-by: David V. Lu <[email protected]>

nav_msgs/msg/GridCells.msg

@@ -3,4 +3,5 @@
 std_msgs/Header header
 float32 cell_width
 float32 cell_height
+# Each cell is represented by the Point at the center of the cell
 geometry_msgs/Point[] cells

nav_msgs/msg/MapMetaData.msg

@@ -13,5 +13,5 @@ uint32 width
 uint32 height
 
 # The origin of the map [m, m, rad].  This is the real-world pose of the
-# cell (0,0) in the map.
+# bottom left corner of cell (0,0) in the map.
 geometry_msgs/Pose origin

nav_msgs/msg/OccupancyGrid.msg

@@ -1,10 +1,13 @@
-# This represents a 2-D grid map, in which each cell represents the probability of occupancy.
-
+# This represents a 2-D grid map
 std_msgs/Header header
 
 # MetaData for the map
 MapMetaData info
 
-# The map data, in row-major order, starting with (0,0).  Occupancy
-# probabilities are in the range [0,100].  Unknown is -1.
+# The map data, in row-major order, starting with (0,0). 
+# Cell (1, 0) will be listed second, representing the next cell in the x direction. 
+# Cell (0, 1) will be at the index equal to info.width, followed by (1, 1).
+# The values inside are application dependent, but frequently, 
+# 0 represents unoccupied, 1 represents definitely occupied, and
+# -1 represents unknown. 
 int8[] data