bug#51824: 27.1; Also have verbose names for gnus-summary-line-format variables

["積丹尼 Dan Jacobson" <jidanni@jidanni.org>]

Maybe in addition to these obscure codes,
have plain English:
Example:
%x = %{xref}

    gnus-summary-line-format is a variable defined in ‘gnus.el’.
    Its value is
    "%U%R%z%I%(%2t %3{%~(cut-left 2)~(max-right 6)~(pad 6)o%}%3L|%6us|%-22,22f%)%3{%un%}:%s
    "
    Original value was 
    "%U%R%z%I%(%[%4L: %-23,23f%]%) %s
    "

      You can customize this variable.

    Documentation:
    The format specification of the lines in the summary buffer.

    It works along the same lines as a normal formatting string,
    with some simple extensions.

    %N          Article number, left padded with spaces (string)
    %S          Subject (string)
    %s          Subject if it is at the root of a thread, and ""
                otherwise (string)
    %n          Name of the poster (string)
    %a          Extracted name of the poster (string)
    %A          Extracted address of the poster (string)
    %F          Contents of the From: header (string)
    %f          Contents of the From: or To: headers (string)
    %x          Contents of the Xref: header (string)
    ...