Minor corrections/additions to VC build instructions

Most of the changes in here are fairly minor.

It is unclear which of the solution files should be used for a 32bit Release build. The solution ``cvisual27.sln`` contains both 32 and 64 bit configurations but the 32bit release configuration puts ``cvisual.pyd`` into ``site-packages\vis\`` rather than ``site_packages\visual_common``.

The file ``VCBuild/cvisual-32bit.vcproj`` puts ``cvisual.pyd`` in the right place (``visual_common``) so I assumed it was the right one (and it worked).

There is no solution (``.sln``) file for it, which raises a warning when compiling form the command line (though it compiles just fine).