qat.4: Minor tweaks

• Document a constraint on the AAD size for AES-GCM.
• Note that the list of supported platforms and add-on devices is not complete and indicate that QAT devices will show up in pciconf output. [1]

PR: 252984 [1]
Sponsored by: Rubicon Communications, LLC ("Netgate")

(cherry picked from commit e1b50e8184fca00520774d43bd7bdd0ccbe9a1d2)