crypto/aes: more precise description for AES keys requirements
The existing documentation is not certain in the place regarding requirements about AES key.
Added some notes for precise description.
Change-Id: I190562ab7c1566cce8e7771f9927d738c72880ce
GitHub-Last-Rev:
6565a8f4e5b37220fd14d55e876b809b2d763b7c
GitHub-Pull-Request: golang/go#71589
Reviewed-on: https://go-review.googlesource.com/c/go/+/647336
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Jorropo <jorropo.pgm@gmail.com>
Auto-Submit: Jorropo <jorropo.pgm@gmail.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>