Merge topic 'HelpEnv'

a693da21f3 Help: from command:if, link to environment variables
ab2bdbaf31 Help: Cross-reference ENV operator from cmake-language(7) manual
ee4e728a69 Help: From Environment Variables section, link to cmake -E env etc

Acked-by: Kitware Robot <kwrobot@kitware.com>
Merge-request: !7239