Corrects equation that calculates dispersion start

The variables were not set, so it could cause some difficulties for
someone not familiar with this.
Fix equation arguments.

When toggling to see the equation without the `>>>`, some arguments
vanished.
Clarify header keywords

I added simple comments to each keyword of the header used to build the
wcs.
Merge pull request #107 from gabraganca/fix_doc_readfits

Fix `read_fits` documentation.