ledger-extra issues

[Werner LEMBERG]

http://lilypond.org/doc/v2.17/Documentation/notation/modifying-single-staves#staff-symbol

This snippet, taken from the documentation (section `Modifying single
staves'):

  \new Staff \with {
    \override StaffSymbol.ledger-extra = #4
  }
  \relative c'' { f'4 a, d, f, }

produces the attached output.  Three issues:

. The documentation confuses me.  It's not clear to me what value 4
  really does.

. The last note has a ledger line at the same position as the lowest
  staff line.  This looks incorrect.

. The topmost ledger line is cut off in the documentation's PNG image,
  something which is essential to retain...


    Werner