Sigstore, Spx: Improve docstring phrasing

No need to talk about TUF specification here: the point is
that we're not sure if the key formats are final and have community
consensus yet -- wherever that may form.

securesystemslib/signer/_sigstore_signer.py

@@ -28,10 +28,9 @@
 class SigstoreKey(Key):
     """Sigstore verifier.
 
-    NOTE: The Sigstore key and signature metadata formats are not part of the
-    TUF specification and are not considered stable in securesystemslib. They
-    may change in future releases and may not be supported by other
-    implementations.
+    NOTE: The Sigstore key and signature serialization formats are not yet
+    considered stable in securesystemslib. They may change in future releases
+    and may not be supported by other implementations.
     """
 
     DEFAULT_KEY_TYPE = "sigstore-oidc"
@@ -90,10 +89,9 @@ def verify_signature(self, signature: Signature, data: bytes) -> None:
 class SigstoreSigner(Signer):
     """Sigstore signer.
 
-    NOTE: The Sigstore key and signature metadata formats are not part of the
-    TUF specification and are not considered stable in securesystemslib. They
-    may change in future releases and may not be supported by other
-    implementations.
+    NOTE: The Sigstore key and signature serialization formats are not yet
+    considered stable in securesystemslib. They may change in future releases
+    and may not be supported by other implementations.
 
     All signers should be instantiated with ``Signer.from_priv_key_uri()``.
     Unstable ``SigstoreSigner`` currently requires opt-in via

securesystemslib/signer/_spx_signer.py

@@ -40,10 +40,9 @@ def generate_spx_key_pair() -> Tuple[bytes, bytes]:
 class SpxKey(Key):
     """SPHINCS+ verifier.
 
-    NOTE: The SPHINCS+ key and signature metadata formats are not part of the
-    TUF specification and are not considered stable in securesystemslib. They
-    may change in future releases and may not be supported by other
-    implementations.
+    NOTE: The SPHINCS+ key and signature serialization formats are not yet
+    considered stable in securesystemslib. They may change in future releases
+    and may not be supported by other implementations.
     """
 
     DEFAULT_KEY_TYPE = "sphincs"
@@ -95,10 +94,9 @@ def verify_signature(self, signature: Signature, data: bytes) -> None:
 class SpxSigner(Signer):
     """SPHINCS+ signer.
 
-    NOTE: The SPHINCS+ key and signature metadata formats are not part of the
-    TUF specification and are not considered stable in securesystemslib. They
-    may change in future releases and may not be supported by other
-    implementations.
+    NOTE: The SPHINCS+ key and signature serialization formats are not yet
+    considered stable in securesystemslib. They may change in future releases
+    and may not be supported by other implementations.
 
     Usage::