Help: Fix CMAKE_SYSROOT doc formatting

Use ``inline-literal`` markup to ensure the --sysroot flag renders
correctly.  Do not link to CMAKE_TOOLCHAIN_FILE variable documentation
that does not yet exist.
Help: Do not reference non-existent CMAKE_TOOLCHAIN_FILE docs
Merge topic 'cross-compiling-toolchain-variables' into next

ab7a69b Help: Do not reference non-existent CMAKE_TOOLCHAIN_FILE docs
eea0276 Help: Fix CMAKE_SYSROOT doc formatting