doc: signature: update algorithms support description

U-Boot supports more hash and verification algorithms these days.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/uImage.FIT/signature.txt b/doc/uImage.FIT/signature.txt
index c71280b..bc123f5 100644
--- a/doc/uImage.FIT/signature.txt
+++ b/doc/uImage.FIT/signature.txt
@@ -42,8 +42,8 @@
 Algorithms
 ----------
 In principle any suitable algorithm can be used to sign and verify a hash.
-At present only one class of algorithms is supported: SHA1 hashing with RSA.
-This works by hashing the image to produce a 20-byte hash.
+U-Boot supports a few hashing and verification algorithms. See below for
+details.
 
 While it is acceptable to bring in large cryptographic libraries such as
 openssl on the host side (e.g. mkimage), it is not desirable for U-Boot.