Update latex-l2tabu.cwl and a bit more

[muzimuzhi]

Follow-up to #4676.

• update latex-l2tabu.cwl
  • remove \fussy, not mentioned in l2tabu.pdf v2.4 (2016-02-03)
  • remove \begin{fussypar} env, non-existing and not-mentioned
  • add a comment for \begin{appendix} env
• update latex-document.cwl
  • mark \begin{sloppypar} as unusual (#*), following \sloppy#*

[muzimuzhi]

With this PR, all entries in latex-l2tabu.cwl but \centerline are recorded in latex-document.cwl.

Current state of \centerline in cwl files:

$ rg -F '\centerline' completion
completion/plaintex.cwl
114:\centerline{text}

completion/tex.cwl
462:# \centerline, in "latex-l2tabu.cwl"

completion/latex-l2tabu.cwl
15:\centerline{text}