[API reference] add some examples to help user understand

[tink2123]

背景：
阅读API reference时，发现有部分内容较简略，仅阅读文档并不清楚这个 API 的作用。因此希望可以为它们增加示例，帮助使用者理解。

修改如下：

1. stack
   文字描述较为抽象，且没有说明不接受 int 数据
   

增加例子：

*case 0:
  Given 
    x = [ [1.0 , 2.0 ] ]
    y = [ [3.0 , 4.0 ] ]
    z = [ [5.0 , 6.0 ] ]
  and
    axis = 0 
  output is:
   out = [ [ [1.0, 2.0] ],
           [ [3.0, 4.0] ],
           [ [5.0, 6.0] ] ]


*case 1:
  Given 
    x = [ [1.0 , 2.0 ] ]
    y = [ [3.0 , 4.0 ] ]
    z = [ [5.0 , 6.0 ] ]
  and
    axis = 1 or axis = -2
  output is:
   out = [ [ [1.0,2.0]
             [3.0,4.0]
             [5.0,6.0] ] ]

*case 2:
  Given 
    x = [ [1.0 , 2.0 ] ]
    y = [ [3.0 , 4.0 ] ]
    z = [ [5.0 , 6.0 ] ]
  and
    axis = 2 or axis = -1
  output is:
   out = [ [ [1.0 ,3.0 ,5.0 ],
             [2.0 ,4.0 ,6.0 ] ] ]

2. multiplex

文字描述较为抽象

增加例子：

*case 0 :
 
Given :

  X =  [  [ [0,0,3,4], [0,1,3,4], [0,2,4,4], [0,3,3,4] ] ,
          [ [1,0,3,4], [1,1,7,8], [1,2,4,2], [1,3,3,4] ] ,
          [ [2,0,3,4], [2,1,7,8], [2,2,4,2], [2,3,3,4] ] ,
          [ [3,0,3,4], [3,1,7,8], [3,2,4,2], [3,3,3,4] ]  ]

    index = [3,0,1,2]

out: 
     [ [3 0 3 4]    // X[3,0] (3 = index[ i ] , 0 = i )  ; i=0
       [0 1 3 4]    // X[0,1] (0 = index[ i ],  1= i )   ; i=1
       [1 2 4 2]    // X[1,2] (0 = index[ i ],  2= i )   ; i=2
       [2 3 3 4] ]  // X[2,3] (0 = index[ i ],  3= i )   ; i=3

*case 1:

Given :

  X =  [  [ [0,0,3,4], [0,1,3,4], [0,2,4,4], [0,3,3,4] ] ,
          [ [1,0,3,4], [1,1,7,8], [1,2,4,2], [1,3,3,4] ]   ]

    index = [1,0]

out: 
     [ [1 0 3 4]    // X[1,0] (3 = index[0], 0 = i )  ; i = 1
       [0 1 3 4]    // X[0,1] (0 = index[1], 1 = i )  ; i = 2
       [0 2 4 4]    // X[0,2] (0 = 0, 2 = i ) ; i = 3
       [0 3 3 4] ]  // X[0,3] (0 = 0, 3 = i ) ; i = 4