Christian Couder [off-list ref] writes:

quoted

  This means that the trimmed <token> and <value> will be separated by
-`': '` (one colon followed by one space).
+`': '` (one colon followed by one space). If the <token> should have a  
different
+string representation than itself, then the 'key' can be configured with
+'trailer.<token>.key'.

I would rather say something like:

"A <token> can be a shortcut name, for example "sign", instead of the
full string, for example "Signed-off-by", which should appear before
the separator on the output. This can be configured using the
'trailer.<token>.key' configuration variable."

Updated in v4 with a slightly different wording versus your example.