docs: Add note to cpu_family table about endianess [skip ci]

Autotools will add endianess to the name of the architecture in some
cases (such as ppc64le vs ppc64) meson doesn't do this. It's worth
noting this in the documentation.

docs/markdown/Reference-tables.md

@@ -65,6 +65,11 @@ set in the cross file.
 Any cpu family not listed in the above list is not guaranteed to
 remain stable in future releases.
 
+Those porting from autotools should note that meson does not add
+endianness to the name of the cpu_family. For example, autotools
+will call little endian PPC64 "ppc64le", meson will not, you must
+also check the `.endian()` value of the machine for this information.
+
 ## Operating system names
 
 These are provided by the `.system()` method call.