Update 02_output.md (#2263)

Added additional documentation to clearly answer the question asked in Issue #2084.

doc/02_output.md

@@ -71,7 +71,7 @@ The output of the JSON files consist of a set of keypoints, whose ordering is re
 There are 2 alternatives to save the OpenPose output. But both of them follow the keypoint ordering described in the section [Keypoint Ordering in C++/Python](#body-keypoints-in-c-python) section (which you should read next).
 
 1. The `--write_json` flag saves the people pose data into JSON files. Each file represents a frame, it has a `people` array of objects, where each object has:
-    1. `pose_keypoints_2d`: Body part locations (`x`, `y`) and detection confidence (`c`) formatted as `x0,y0,c0,x1,y1,c1,...`. The coordinates `x` and `y` can be normalized to the range [0,1], [-1,1], [0, source size], [0, output size], etc. (see the flag `--keypoint_scale` for more information), while the confidence score (`c`) in the range [0,1].
+    1. `pose_keypoints_2d`: Body part locations (`x`, `y`) and detection confidence (`c`) formatted as `x0,y0,c0,x1,y1,c1,...`. The coordinates `x` and `y` can be normalized to the range [0,1], [-1,1], [0, source size], [0, output size], etc. (see the flag `--keypoint_scale` for more information), while the confidence score (`c`) in the range [0,1]. Additionally, the number of keypoints output in this array will remain consistent between JSON files.
     2. `face_keypoints_2d`, `hand_left_keypoints_2d`, and `hand_right_keypoints_2d` are analogous to `pose_keypoints_2d` but applied to the face and hand parts.
     3. `body_keypoints_3d`, `face_keypoints_3d`, `hand_left_keypoints_2d`, and `hand_right_keypoints_2d` are analogous but applied to the 3-D parts. They are empty if `--3d` is not enabled. Their format is `x0,y0,z0,c0,x1,y1,z1,c1,...`, where `c` is 1 or 0 depending on whether the 3-D reconstruction was successful or not.
     4. `part_candidates` (optional and advanced): The body part candidates before being assembled into people. Empty if `--part_candidates` is not enabled (see that flag for more details).