Update parallel-computing.rst

minor edits in first example in parallel-computing.rst to switch from deprecated usage of '+' to '.+' for scalar-array addition.
Reintroduce nnz and nonzeros as discussed in #6769
Deprecate nfilled for SparseMatrixCSC.
Deprecate nonzeros for StridedArray and BitArray.
Deprecate nnz for StridedArray.
Update documentation for structural nonzeros.
Update NEWS