org_mapping docs update proposition

[clementduveau]

Hello, I noticed a bunch of small improvments for the docs

1. org_mapping

org_mapping (String) List of comma- or space-separated Organization:OrgId:Role mappings. Organization can be * meaning “All users”. Role is optional and can have the following values: Viewer, Editor or Admin.

We need to add a warning for Grafana Cloud: it's restricted.
Also, the None role is available

2. OAuth2 in SAML

client_id & client_secret in the SAML section refer to OAuth2

I can do it later, just taking notes