[mlir][sparse] Rename the public SparseTensorStorage::asCOO to toCOO

Trying to reduce confusion by having the name of the public method match that of the private method for handling the recursion.  Also adding some comments to SparseTensorStorage::fromCOO to help clarify what the recursive calls are doing in the dense case.

Reviewed By: aartbik

Differential Revision: https://reviews.llvm.org/D108954