Integrated generalized-void.md into dartLangSpec.tex

Also marked the feature specification generalized-void.md
as background material

Change-Id: I08c9b54d956208db5aa8e695540b0c1fc941e46b
Reviewed-on: https://dart-review.googlesource.com/c/86421
Reviewed-by: Lasse R.H. Nielsen <[email protected]>

Author: eernstg