Update documentation of vulnerability function distribution types [skip CI]
Update documentation of vulnerability function distribution types [skip CI]

Mention that the Beta distribution and discrete distributions are also supported

Former-commit-id: ab0389d642fc6cb467e00c439da989dab37c6da4
gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine