Merge topic 'doc-vs-toolset-options'

3df2506f Help: Document VS generator toolset specification syntax
60546b48 Help: Cross-reference generator toolset variables
35c5cddc Help: Cross-reference generator platform variables
7df38939 Help: Move generator toolset support details to variable docs
c750b282 Help: Move generator platform support details to variable docs

Acked-by: Kitware Robot <kwrobot@kitware.com>
Merge-request: !580