CMAKE_TOOLCHAIN_FILE: Document relative path behavior

Help/variable/CMAKE_TOOLCHAIN_FILE.rst

@@ -7,3 +7,6 @@ This variable is specified on the command line when cross-compiling with CMake.
 It is the path to a file which is read early in the CMake run and which
 specifies locations for compilers and toolchain utilities, and other target
 platform and compiler related information.
+
+Relative paths are allowed and are interpreted first as relative to the
+build directory, and if not found, relative to the soruce directory.