[ADFS docs] Clarify private key/cert usage

[ryanelliottsmith]

Clarify what the private key/certificate is used for in ADFS

For Rancher (product) docs only

When contributing to docs, please update the versioned docs. For example, the docs in the v2.6 folder of the rancher folder.

Doc versions older than the latest minor version should only be updated to fix inaccuracies or make minor updates as necessary. The majority of new content should be added to the folder for the latest minor version.