Residual SIMD Doxygen improvements

- Add a link to the overview documentation from the module page.
- Remove some trailing spaces.

Some issues noted, but not fixed here:
- \nosubgrouping would make the file pages look more consistent, if it
  worked for files...
- Some of the SIMD routines are not included in any named group, making
  them appear separately at the end of the documentation.
- The list of members on the SIMD module page is exceedingly long,
  making it hard to find things there.  Would require some thinking on
  how to best solve this.

Change-Id: I628f321172a3c3e8f4874e166c48e12763d1490e