doc: clarify openssl-based key and certificate generation process

Add some basic clarification that the dev.key file generated by OpenSSL
contains both the public and private key, and further highlight that
the certificate generated here contains the public key only.

Signed-off-by: Andreas Dannenberg <dannenberg@ti.com>

doc/uImage.FIT/signature.txt

@@ -62,14 +62,14 @@ placed alongside rsa.c, and its functions added to the table in image-sig.c
 also.
 
 
-Creating an RSA key and certificate
------------------------------------
-To create a new public key, size 2048 bits:
+Creating an RSA key pair and certificate
+----------------------------------------
+To create a new public/private key pair, size 2048 bits:
 
 $ openssl genpkey -algorithm RSA -out keys/dev.key \
     -pkeyopt rsa_keygen_bits:2048 -pkeyopt rsa_keygen_pubexp:65537
 
-To create a certificate for this:
+To create a certificate for this containing the public key:
 
 $ openssl req -batch -new -x509 -key keys/dev.key -out keys/dev.crt