[DOC] Documentation for builtin gnmf function
[DOC] Documentation for builtin gnmf function

  Example:
  ```dml
  X = rand(rows = 5, cols = 3)
  [A, B] = gnmf(X = X, rnk = 2, eps = 10^-8, maxi = 10)
  ```

  Matrix A (Pattern matrix):
  0.066 -0.068
  0.003 -0.270
  0.113 -0.007
  0.087 -0.035
  0.077 -0.040

  Matrix B (Amplitude matrix):
  1.043 6.335 7.038
  -1.589 -0.115 -0.608

Closes #941.

apache/incubator-systemml,apache/incubator-systemml,apache/incubator-systemml,apache/incubator-systemml,apache/incubator-systemml,apache/incubator-systemml