cpp_info.sysroot documentation is missing

[djinni-developer]

Following up on a slack conversation, cpp_info.sysroot is not documented. I suspect it is a useful feature for toolchain packages, so I think at a minimum, the documentation should include the following:

• description of cpp_info.sysroot
• updating cross-building examples to use sysroot
• a mapping of cpp_info.sysroot to generator variables