ARROW-4335: [C++] Better document sparse tensor support
ARROW-4335: [C++] Better document sparse tensor support

I wrote descriptions for sparse tensor classes.

Author: Kenta Murata <mrkn@mrkn.jp>

Closes #3810 from mrkn/sparse_tensor_doc and squashes the following commits:

9c4fe1422 <Kenta Murata> Fix incorrect English
522a982f1 <Kenta Murata> Modify comments following review comments
83cab0377 <Kenta Murata>  Put SparseIndexBase<> class in internal namespace
6cd13e2eb <Kenta Murata>  Write descriptions of sparse tensor

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