doc: interpret-trailers: add key format example

All of the examples speak of the Happy Path where everything works
as intended. But failure examples can also be instructive. Especially
for explaining again, by example, the key format (see previous commit).

This also allows us to demonstrate trailer block detection with a
concrete example.

Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-interpret-trailers.adoc

@@ -405,6 +405,29 @@ mv "\$1.new" "\$1"
 $ chmod +x .git/hooks/commit-msg
 ------------
 
+* Here we try to use three different trailer keys. But it fails because
+  two of them are not recognized as trailer keys.
++
+----
+$ cat msg.txt
+subject
+
+Skapad-på: some-branch
+Hash-in-v6.11: 45c12d3269fe48f22834320c782ffe86c3560f2c
+Reviewed-by: Alice <alice@example.com>
+$ git interpret-trailers --only-trailers <msg.txt
+$
+----
++
+Recall that a trailer key has to consist of only ASCII alphanumeric
+characters and hyphens, and this does not hold for the two first
+supposed trailer keys. And now none are recognized as trailers because
+the candidate trailer block has at least one non-trailer line, even
+though `Reviewed-by` is a valid trailer key. Recall that a trailer block
+has to either (i) be all trailers, or (ii) consist of at least one
+Git-generated or user-configured trailer (and some other conditions).
+And (ii) is not satisfied since we have not configured any trailer keys.
+
 SEE ALSO
 --------
 linkgit:git-commit[1], linkgit:git-format-patch[1], linkgit:git-config[1]