doc bash 2-5, for pack, unpack, pack_img and unpack_img (#6140)

* doc bash for pack, unpack, pack_img and unpack_img

* Add comments for labels could be 1d list

* Update recordio.py

* Update recordio.py

* Update recordio.py

fixing text

* Update recordio.py

fixing text

* remove empty line

python/mxnet/recordio.py

@@ -296,9 +296,23 @@ def pack(header, s):
     ----------
     header : IRHeader
         Header of the image record.
-        ``header.label`` can be a number or an array.
+        ``header.label`` can be a number or an array. See more detail in ``IRHeader``.
     s : str
-        string to pack
+        Raw image string to be packed.
+
+    Returns
+    -------
+    s : str
+        The packed string.
+
+    Examples
+    --------
+    >>> label = 4 # label can also be a 1-D array, for example: label = [1,2,3]
+    >>> id = 2574
+    >>> header = mx.recordio.IRHeader(0, label, id, 0)
+    >>> with open(path, 'r') as file:
+    ...     s = file.read()
+    >>> packed_s = mx.recordio.pack(header, s)
     """
     header = IRHeader(*header)
     if isinstance(header.label, numbers.Number):
@@ -324,6 +338,14 @@ def unpack(s):
         Header of the image record.
     s : str
         Unpacked string.
+
+    Examples
+    --------
+    >>> record = mx.recordio.MXRecordIO('test.rec', 'r')
+    >>> item = record.read()
+    >>> header, s = mx.recordio.unpack(item)
+    >>> header
+    HEADER(flag=0, label=14.0, id=20129312, id2=0)
     """
     header = IRHeader(*struct.unpack(_IR_FORMAT, s[:_IR_SIZE]))
     s = s[_IR_SIZE:]
@@ -340,14 +362,34 @@ def unpack_img(s, iscolor=-1):
     s : str
         String buffer from ``MXRecordIO.read``.
     iscolor : int
-        image format option for ``cv2.imdecode``.
+        Image format option for ``cv2.imdecode``.
 
     Returns
     -------
     header : IRHeader
         Header of the image record.
     img : numpy.ndarray
         Unpacked image.
+
+    Examples
+    --------
+    >>> record = mx.recordio.MXRecordIO('test.rec', 'r')
+    >>> item = record.read()
+    >>> header, img = mx.recordio.unpack_img(item)
+    >>> header
+    HEADER(flag=0, label=14.0, id=20129312, id2=0)
+    >>> img
+    array([[[ 23,  27,  45],
+            [ 28,  32,  50],
+            ...,
+            [ 36,  40,  59],
+            [ 35,  39,  58]],
+           ...,
+           [[ 91,  92, 113],
+            [ 97,  98, 119],
+            ...,
+            [168, 169, 167],
+            [166, 167, 165]]], dtype=uint8)
     """
     header, s = unpack(s)
     img = np.fromstring(s, dtype=np.uint8)
@@ -362,9 +404,9 @@ def pack_img(header, img, quality=95, img_fmt='.jpg'):
     ----------
     header : IRHeader
         Header of the image record.
-        ``header.label`` can be a number or an array.
+        ``header.label`` can be a number or an array. See more detail in ``IRHeader``.
     img : numpy.ndarray
-        image to pack
+        Image to be packed.
     quality : int
         Quality for JPEG encoding in range 1-100, or compression for PNG encoding in range 1-9.
     img_fmt : str
@@ -374,6 +416,14 @@ def pack_img(header, img, quality=95, img_fmt='.jpg'):
     -------
     s : str
         The packed string.
+
+    Examples
+    --------
+    >>> label = 4 # label can also be a 1-D array, for example: label = [1,2,3]
+    >>> id = 2574
+    >>> header = mx.recordio.IRHeader(0, label, id, 0)
+    >>> img = cv2.imread('test.jpg')
+    >>> packed_s = mx.recordio.pack_img(header, img)
     """
     assert cv2 is not None
     jpg_formats = ['.JPG', '.JPEG']