cpp_info.sysroot documentation is missing

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

Start by locating the documentation for cpp_info and the existing cross-building examples. Document cpp_info.sysroot, update the relevant examples to use it, and add the requested mapping to generator variables; the issue is complete when all three areas are covered.

Written by the indexing model from the issue text.