[Bug-readline] docs of history_get()

[Kevin Ryde]

In the readline 7.0 info docs, I found the description of history_get()
a little unclear.  It read to me at first like the OFFSET parameter was
an offset relative to history_base, so 0 up to history length or
something.  If I'm not mistaken that's so of where_history(), set_pos()
etc, but history_get() is a full event number.  Maybe something like,
assuming its correct :),

 -- Function: HIST_ENTRY * history_get (int n)
     Return the history entry at absolute event designator N.  Entries
     are from oldest at 'history_base' (*note History Variables::) to
     newest at 'history_base + history_length - 1'.  If N is outside
     this range (or the history is empty) then return 'NULL'.