Describe the signature scheme in docs/FORMAT.md as the code builds it (closes #187)

The Signature Scheme section now states what signatureString builds: the
magic, the outer uuid as lower-case hex with no hyphens, and the outer
sha256 as the lower-case hex of its multihash, which starts 1220, with an
example string taken from a run of the code. It says signer is the
upper-case hex fingerprint of the signing primary key and that a signature
is stored only in the manifest. The mf.proto comments on fields 201 to 203
say the same, and mf.pb.go is regenerated. A new test pins the example
string. Nothing about what is signed or how changes.

Model: opus-5-5

mfer/mf.proto

@@ -32,15 +32,17 @@ message MFFileOuter {
     bytes uuid = 105;
 
     bytes innerMessage = 199;
-    // 2xx for optional manifest root attributes
-    // think we might use gosignify instead of gpg:
-    // github.com/frankbraun/gosignify
 
-    //detached signature, ascii or binary
+    // 2xx for optional manifest root attributes
+
+    // one OpenPGP signature, armored or binary, over the canonical string
+    // ZNAVSRFG-<uuid>-<sha256 as a multihash>, with the uuid as 32
+    // lower-case hex digits and no hyphens, and the multihash in lower-case
+    // hex (see docs/FORMAT.md). A signature is never stored anywhere else.
     optional bytes signature = 201;
-    //full GPG key id
+    // fingerprint of the signing primary key, in upper-case hex
     optional bytes signer = 202;
-    //full GPG signing public key, ascii or binary
+    // OpenPGP public key of the signer, one primary key, armored or binary
     optional bytes signingPubKey = 203;
 }