Feature request: Show "format controls the output" with --help

[hellishvictor]

Hi, there's some info that is missing using coreutils date --help which I didn't knew it exist until check it the gnuwin32 date.exe:

FORMAT controls the output.  The only valid option for the second form
specifies Coordinated Universal Time.  Interpreted sequences are:

  %%   a literal %
  %a   locale's abbreviated weekday name (Sun..Sat)
  %A   locale's full weekday name, variable length (Sunday..Saturday)
  %b   locale's abbreviated month name (Jan..Dec)
  %B   locale's full month name, variable length (January..December)
  %c   locale's date and time (Sat Nov 04 12:02:33 EST 1989)
  %C   century (year divided by 100 and truncated to an integer) [00-99]
  %d   day of month (01..31)
  %D   date (mm/dd/yy)
  %e   day of month, blank padded ( 1..31)
  %F   same as %Y-%m-%d
  %g   the 2-digit year corresponding to the %V week number
  %G   the 4-digit year corresponding to the %V week number
  %h   same as %b
  %H   hour (00..23)
  %I   hour (01..12)
  %j   day of year (001..366)
  %k   hour ( 0..23)
  %l   hour ( 1..12)
  %m   month (01..12)
  %M   minute (00..59)
  %n   a newline
  %N   nanoseconds (000000000..999999999)
  %p   locale's upper case AM or PM indicator (blank in many locales)
  %P   locale's lower case am or pm indicator (blank in many locales)
  %r   time, 12-hour (hh:mm:ss [AP]M)
  %R   time, 24-hour (hh:mm)
  %s   seconds since `00:00:00 1970-01-01 UTC' (a GNU extension)
  %S   second (00..60); the 60 is necessary to accommodate a leap second
  %t   a horizontal tab
  %T   time, 24-hour (hh:mm:ss)
  %u   day of week (1..7);  1 represents Monday
  %U   week number of year with Sunday as first day of week (00..53)
  %V   week number of year with Monday as first day of week (01..53)
  %w   day of week (0..6);  0 represents Sunday
  %W   week number of year with Monday as first day of week (00..53)
  %x   locale's date representation (mm/dd/yy)
  %X   locale's time representation (%H:%M:%S)
  %y   last two digits of year (00..99)
  %Y   year (1970...)
  %z   RFC-2822 style numeric timezone (-0500) (a nonstandard extension)
  %Z   time zone (e.g., EDT), or nothing if no time zone is determinable

By default, date pads numeric fields with zeroes.  GNU date recognizes
the following modifiers between `%' and a numeric directive.

  `-' (hyphen) do not pad the field
  `_' (underscore) pad the field with spaces

Would be awesome if were implemented. Love this project.
Cheers.