Docs: Improved the docs for writing functions taking Eigen types.
Docs: Improved the docs for writing functions taking Eigen types.

- Removed the wrong statement about the MSVC compiler.
- Reformulated "simple functions" usage.
- Reformulated the summary paragraph about writable parameters.

madlib/eigen_backup,mjbshaw/Eigen,madlib/eigen_backup,rotorliu/eigen,rotorliu/eigen,madlib/eigen_backup,mjbshaw/Eigen,madlib/eigen_backup,rotorliu/eigen,mjbshaw/Eigen,mjbshaw/Eigen,rotorliu/eigen