Key sizes can only be certain lengths

[tkhadimullin]

The wording could be interpreted as "any value between 128 and 256 bits is valid" leading to confision.
Specifying exact values will hopefully provide more clarity and eliminate confusion.
See https://github.com/microsoft/referencesource/blob/master/System.Core/System/Security/Cryptography/AesCryptoServiceProvider.cs#L323 for implementation details.

Summary

Describe your changes here.

Fixes #Issue_Number (if available)

[bartonjs]

It's probably worth checking with AesCng and Aes (the base class) to make sure that they're all equally clear.

[tkhadimullin]

Documentation for Aes itself does not seem to make any representations regarding legal key sizes. The source code actually defines what sizes are valid: https://github.com/microsoft/referencesource/blob/master/mscorlib/system/security/cryptography/aes.cs#L21.
As far as I understand it, the logic is "between 128 and 256 with increments of 64 bits. Which still yields the same 3 key sizes. Arguably this mathematics is implementation details and likely does not belong in documentation?

AesCng actually exposes documentation regarding this property, so I updated it with the same wording.

[tkhadimullin]

was there anything else I could do to move this PR along?

[bartonjs]

@dotnet/docs This is ready for final review