doc addition: IR v2.20.0, 3.1.9 Arpeggio, positions

[Owain Evans]

Hi Bug-Lily,

In IR v2.20.0
3.1.9 Arpeggio, positions (pair of numbers),
http://lilypond.org/doc/v2.20/Documentation/internals/arpeggio
        
Says (left . right). Should read (bottom . top).
Please change to read:
----
Pair of staff coordinates (bottom . top), where both bottom and top are in 
staff-space units of the current staff. For slurs, this value is (left-vertical 
. right-vertical) and selects which slur candidate to use; if extreme positions 
are requested, the closest one is taken.
----
"The for slurs part" I've change so it makes sense, but unsure if 
"(left-vertical . right-vertical)" too verbose or correct naming convention.
        
An example of it so it's clear it's (bottom . top) not (left . right)
\version "2.20.0"
{
  \override Staff.Arpeggio.positions = #'(-4 . 4)
  <f' b' e''>\arpeggio
}
Please could it be corrected.
Many thanks,

Owain Evans