Address Lukas comments for "Add the Signer interface..."

Signed-off-by: Martin Vrachev <mvrachev@vmware.com>

securesystemslib/signer.py

@@ -43,8 +43,7 @@ def from_dict(cls, signature_dict):
       Creates a Signature object from its JSON/dict representation.
 
     <Arguments>
-      signature_dict: A dict containing that should contain valid keyid
-        and signature.
+      signature_dict: A dict that contains a valid keyid and signature.
         Note that the fields should be named "keyid" and "sig" respectively.
     """
 
@@ -83,7 +82,7 @@ def sign(payload):
 
     <Returns>
       Returns a "Signature" class instance containing the signature and the
-      the keyid which uniquely identifies the key used for signature generation.
+      keyid which uniquely identifies the key used for signature generation.
     """
     raise NotImplementedError # pragma: no cover
 
@@ -95,7 +94,8 @@ class SSlibSigner(Signer):
       Securesystemslib default implementation of the "Signer" interface.
       With this implementation the following signature schemes are supported:
 
-      'RSASSA-PSS'
+      'rsassa-pss-(md5|sha1|sha224|sha256|sha384|sha512)'
+      'rsa-pkcs1v15-(md5|sha1|sha224|sha256|sha384|sha512)'
       RFC3447 - RSASSA-PSS
       http://www.ietf.org/rfc/rfc3447.