[SPARK-5958][MLLIB][DOC] update block matrix user guide

* Removed SVD code from examples.
* Corrected Java API doc link.
* Updated variable names: `AtransposeA` -> `ata`.
* Minor changes.

brkyvz

Author: Xiangrui Meng <meng@databricks.com>

Closes #4737 from mengxr/update-block-matrix-user-guide and squashes the following commits:

70f53ac [Xiangrui Meng] update block matrix user guide