-** 'only': omit non-trailer lines from the trailer block.
-** 'unfold': make it behave as if interpret-trailer's `--unfold`
- option was given. E.g., `%(trailers:only,unfold)` unfolds and
- shows all trailer lines.
+** 'only[=val]': select whether non-trailer lines from the trailer
+ block should be included. The `only` keyword may optionally be
+ followed by an equal sign and one of `true`, `on`, `yes` to omit or
+ `false`, `off`, `no` to show the non-trailer lines. If option is
+ given without value it is enabled. If given multiple times the last
+ value is used.
+** 'unfold[=val]': make it behave as if interpret-trailer's `--unfold`
+ option was given. In same way as to for `only` it can be followed
+ by an equal sign and explicit value. E.g.,
+ `%(trailers:only,unfold=true)` unfolds and shows all trailer lines.