docs(psirt): who can use PSIRT, and why the permission beats the role

[devGregA]

Description

Documents who can use PSIRT, alongside a Pro change that adds a PSIRT configuration
permission.

Until now the module admitted a superuser or a global Maintainer/Owner and nobody
else, so the page had nothing to say about access beyond the feature flag and the
licence. With a PSIRT-specific permission available, the docs can say the thing that
actually matters: prefer the permission, because a global Maintainer role is write
access to every asset in the instance — granting it so somebody can read advisories
hands them the rest of the product too.

Covers:

• the three routes in, and which one to prefer;
• the View / Change split, and that View is genuinely read-only;
• that the licence is checked before any of it and is not a permission — with no
  PSIRT Advisory Engine entitlement the module is closed to everyone, superusers
  included;
• that the permission only appears in the picker while the PSIRT feature flag is on.

Docs-only: no code changes.