securejoin: do not check the signatures existance twice

Mimeparser.was_encrypted() checks if the message is an Autocrypt encrypted
message. It already means the message has a valid signature.

This commit documents a few functions to make it clear that signatures
stored in Mimeparser must be valid and must always come from encrypted
messages.

Also one unwrap() is eliminated in encrypted_and_signed(). It is possible
to further simplify encrypted_and_signed() by skipping the was_encrypted()
check, because the function only returns true if there is a matching
signature, but it is helpful for debugging to distinguish between
non-Autocrypt messages and messages whose fingerprint does not match.

src/e2ee.rs

@@ -115,6 +115,12 @@ impl EncryptHelper {
     }
 }
 
+/// Tries to decrypt a message, but only if it is structured as an
+/// Autocrypt message, i.e.  encrypted and signed with a valid
+/// signature.
+///
+/// Returns decrypted body and a set of valid signature fingerprints
+/// if successful.
 pub async fn try_decrypt(
     context: &Context,
     mail: &ParsedMail<'_>,