[mlir][sparse] refactoring SparseTensorUtils: (4 of 4) documentation

Previously, the SparseTensorUtils.cpp library contained a C++ core implementation, but hid it in an anonymous namespace and only exposed a C-API for accessing it. Now we are factoring out that C++ core into a standalone C++ library so that it can be used directly by downstream clients (per request of one such client). This refactoring has been decomposed into a stack of differentials in order to simplify the code review process, however the full stack of changes should be considered together.

* D133462: Part 1: split one file into several
* D133830: Part 2: Reorder chunks within files
* D133831: Part 3: General code cleanup
* (this): Part 4: Update documentation

This part updates existing documentation, adds new documentation, and reflows the test for some existing documentation.

Depends On D133831

Reviewed By: aartbik

Differential Revision: https://reviews.llvm.org/D133833