Doc: harmonics for fretted strings

[Federico Bruni]

Hi folks

I have a few questions about harmonics:

1. \harmonic _must_ be placed inside a chord even for single notes? 
Why? Is it still true?
In NR 2.3.1 Harmonics>Artificial harmonics there's a warning:

"""
\harmonic must be placed inside a chord construct even if there is only 
a single note. Normally \harmonicsOn would be used in this situation.
"""

It does not explain why. I cannot see any difference:

\version "2.19.43"
{
 d'8\harmonic <d'\harmonic>
 d'2\harmonic <d'\harmonic>
}


In NR 2.4.1 it explains that it should not be attached to chords but on 
single notes within the chord. This is true and makes sense:

"""
Note that the command \harmonic must always be attached to single notes 
(possibly inside of a chord) instead of whole chords.
"""


2. Harmonics for fretted strings
In NR 2.4.1, References for fretted strings, instructions for 
indicating harmonics
point to the Harmonics node, which is specific of unfretted strings. 
But there's also
a section about harmonics for fretted strings (\harmonicByFret and 
\harmonicByRatio).
We should link also to this one. The problem is that it's within a long 
chapter,
Default tablatures (so you have to scroll a lot to find it). Unless we 
can link
to the @cindex entry:
http://lilypond.org/doc/v2.19/Documentation/notation/common-notation-for-fretted-strings#index-harmonic-indications-in-tablature-notation


3. Explain better \harmonicByFret and \harmonicByRatio
Even though examples show well how they work, I would add a syntax 
explanation, like:

\harmonicByFret #FRET STRING-NOTE\STRING-NUMBER

Important: explain that if you use these functions within a \relative, 
the
note following this function will be relative to the _sounding pitch_ 
calculated
by LilyPond. See this example:

\version "2.19.43"

myMusic = {
 d \harmonicByFret #5 d\4 d
 % in relative mode the last d is relative to the harmonic sounding 
pitch
 \relative { d \harmonicByFret #5 d\4 d }
}

\score {
 \new StaffGroup <<
   \new Staff { \clef "treble_8" \myMusic }
   \new TabStaff { \clef "moderntab" \new TabVoice \myMusic }
 >>
 \layout {
   indent = 0
   \omit StringNumber
 }
}