document that .append also works when the axis already exists + deprecate .extend

[gdementen]

>>> arr = ndtest(3)
>>> arr
a  a0  a1  a2
    0   1   2
>>> arr.append('a', 42, 'a3')
a  a0  a1  a2  a3
    0   1   2  42
>>> arr2 = Array([42], 'a=a3')
>>> arr.extend('a', arr2)
a  a0  a1  a2  a3
    0   1   2  42
>>> arr3 = Array([42, 43], 'a=a3,a4')
>>> arr.extend('a', arr3)
a  a0  a1  a2  a3  a4
    0   1   2  42  43
>>> arr.append('a', arr3)
a  a0  a1  a2  a3  a4
    0   1   2  42  43

[gdementen]

FWIW, I did a little poll about this for our heaviest users. So far the results are:
jd +1, rd +1, bm -0, gb +0
So I will do this.