[bug-mailutils] date format and documentation

[積丹尼 Dan Jacobson]

On (info "(mailutils) Mail Variables")

     Letter         Meaning
     --------------------------------------------------------------------------
     %a             Message attributes.
     %d             The date when the message was received.  **mention *Note 
datefield:: here too**
     %f             The address of the message sender.
     %l             The number of lines of the message.
     %m             Message number.
     %o             The number of octets (bytes) in the message.

Also here you should also have date(1) % modifiers, as one would
probably not want a one size fits all date style!