Add some guidelines on the naming of the environment in the documentation

[pmalarme]

Name of the environment can be only alphanumeric and hyphen. Add it to the documentation.

This issue is for a: (mark with an x)

- [ ] bug report -> please search issues before submitting
- [ ] feature request
- [x] documentation issue or request
- [ ] regression (a behavior that used to work and stopped in a new release)