Document CMAKE_DISABLE_SOURCE_CHANGES and CMAKE_DISABLE_IN_SOURCE_BUILD
As suggested on https://stackoverflow.com/a/10306476, these variables are useful to avoid accidentally writing into the source tree, but since they are undocumented there is concern that they may change without warning in future versions. Looking at the git history, the syntax hasn't changed since introduced in 2006, so perhaps it would be worth documenting this feature.