add argument to help history

[積丹尼 Dan Jacobson]

$ help history
      -d offset delete the history entry at offset OFFSET.
      -a        append history lines from this session to the history file
      -n        read all history lines not already read from the history file
      -r        read the history file and append the contents to the history 
list
      -w        write the current history to the history file and append them 
to the history list

Item -d above lists its argument but the others don't. The others should
say
      -a [filename] append history lines from this session to the history file
      -n [filename] read all history lines not already read from the history 
file
      -r [filename] read the history file and append the contents to the 
history list
      -w [filename] write the current history to the history file and append 
them to the history list