Add more explanation to spec.description for bundle gem template.

"Optional" sounds like you can leave this value unchanged, but will
raise an exception in this case, aborting before the gem is built:
```
WARNING:  See http://guides.rubygems.org/specification-reference/ for help
ERROR:  While executing gem ... (Gem::InvalidSpecificationException)
  "FIXME" or "TODO" is not a description
```