[Documentation] Discourage using CMAKE_SOURCE_DIR or CMAKE_BINARY_DIR in toolchain file
CMAKE_SOURCE_DIR and CMAKE_BINARY_DIR will be different for project and try_compile invocations. For toolchain files where toolchain itself is downloaded or other heavy tasks are performed it will cause toolchain to be extracted/deleted numerous times.
For cases where the user wants to include other cmake files they won't be there, e.g. see #16744 (closed)
Let's discourage using these variables in documentation for CMAKE_TOOLCHAIN_FILE.
If someone could point out doc source file I'd do merge request myself.