[lltx] luatextra update

[Manuel Pégourié-Gonnard]

Hi folks,

I just had a small burst of activity and updated a few packages, doing mainly
documentation fixes. Most notably, I added a section "planned changes" to
luatextra.pdf, concerning planned incompatible changes in the next release.
Please read it if you're interested and tell me if you disagree.

There are a few incompatible changes pending in luatexbase-modutils, but I'll
discuss them later (you can have a look at the "unstable" branch on github if
you're curious).

I also removed luatextra-reference which had become to messy, but I'll probably
revive it as an independant document lualatex-doc.pdf.

Manuel.