#5950 Doc: mention tuplet-slur in Changes and in NR 1.2.1.2 "Tuplets"

Doc: mention tuplet-slur in Changes and in NR 1.2.1.2 "Tuplets"

See issue #5567 for David Nalesnik’s original patch.
Documenting why the bracket-visibility default behavior
isn’t suited to tuplet-slurs allows me to explicitely
point towards the relevant snippet for more clarity,
and to add a couple of additional internal references.

https://codereview.appspot.com/559930043

[Anonymous]

Passes make, make check and a full make doc.

[Anonymous]

Patch on countdown for May 6th - nits from Werner on Rietveld

[Anonymous]

Patch counted down - please push - but see the nits on Rietveld Valentin.

[Valentin Villenave]

Pushed as https://git.savannah.gnu.org/cgit/lilypond.git/commit/?id=ee56da0f7a68cb6d6c5cd8531a566d81abdc5e59