Fix make -C Documentation po-update.

Fix make -C Documentation po-update.

Use --keyword=_doc instead of running SED on copied sources.
Add warning message in texi-langutils.py:

  * Why do we need node names extracted into Documentation/po
    and translated?  That's already done inline using @translationof?
  * Why do we need translated node names of ALL languages there?

I'm tempted to junk that and add

    -        elif output_file:
    +        elif output_file and scan_ly:

in texi-langutils.py

I.e., why do we have

    #. @node in Documentation/learning/tutorial.itely
    #. @section in Documentation/learning/tutorial.itely
    msgid "How to write input files"
    msgstr ""

when this node is translated inline in
Documentation/<LANG>/learning/tutorial.itely?

And why are these?

    #. @node in Documentation/fr/essay.tely
    #. @appendix in Documentation/fr/essay.tely
    #. @node in Documentation/fr/learning.tely
    #. @appendix in Documentation/fr/learning.tely
    #. @node in Documentation/fr/notation.tely
    #. @appendix in Documentation/fr/notation.tely
    #. @node in Documentation/fr/usage.tely
    #. @appendix in Documentation/fr/usage.tely
    msgid "Index de LilyPond"
    msgstr ""

    #. @subsubheading in Documentation/fr/macros.itexi
    msgid "Commandes prédéfinies"
    msgstr ""