DOC: Fix reference warning for buffer.

[liang3zy22]

Fix following warning:

arrays.dtypes.rst:187: WARNING: py:class reference target not found: buffer
arrays.interface.rst:181: WARNING: py:class reference target not found: buffer
arrays.ndarray.rst:24: WARNING: py:class reference target not found: buffer

I searched to find that buffer is replaced by memoryview. So I used memoryview class reference.

[mattip]

Thanks @liang3zy22

[mattip]

I used this one-liner on the build log to find the sources of the 159 remaining warnings

grep "reference target not found" build.log | sed -e "s/reference target not found/z/" | cut -f2- -dz | sort | uniq -c | sort -n

Here are the references with more than one error. It would be nice to solve the size one, I think that comes from a docstring for a function with a size parameter. Somehow that particular kwarg messes up sphinx/numpydoc.

      2 : CCompiler
      2 : doc.indexing
      2 : index_exp
      2 : ndim
      2 : npy_datetime
      2 : PyArray_VectorUnaryFunc
      3 : FILE
      3 : formatter
      3 : numpy.core.defchararray
      3 : numpy.ma.core.MaskedArray
      3 : PyGenericArr_Type
      5 : ABCPolyBase
      6 : trimseq
      9 : size_t
     51 : size

[liang3zy22]

@mattip , OK, let me do it.