Expand description of UseX509Certificate

[josephdecock]

Folks are often confused by the fact that the certificate that wraps the key material can have an expiration that isn't relevant to us. We should clarify that.

Existing places where we reference UseX509Certificate:

• https://docs.duendesoftware.com/identityserver/v7/reference/options/
• https://docs.duendesoftware.com/identityserver/v7/fundamentals/keys/automatic_key_management/