Make wording clearer for payload signature verification (#5002)

Currently wordings for signature verification for p2p
`execution_payload` makes sense for non self-build scenario but is
confusing for self-build scenario. Update wordings such that the rule
works for both.

Author: ensi321

specs/gloas/p2p-interface.md

@@ -294,8 +294,8 @@ obtained from the `state.latest_execution_payload_bid`)
 - _[REJECT]_ `block.slot` equals `envelope.slot`.
 - _[REJECT]_ `envelope.builder_index == bid.builder_index`
 - _[REJECT]_ `payload.block_hash == bid.block_hash`
-- _[REJECT]_ `signed_execution_payload_envelope.signature` is valid with respect
-  to the builder's public key.
+- _[REJECT]_ `signed_execution_payload_envelope.signature` is valid as verified
+  by `verify_execution_payload_envelope_signature`.
 
 ###### `payload_attestation_message`