Document autocoded serialization and deserialization

[zimri-leisher]

The behavior of the autocoded ser/deser functions for structs/arrays/enums should be well-defined and guaranteed not to change.
As discussed with @bocchino , for serialization, the definition should be:

1. struct members are serialized in the order defined in fpp
2. no padding
3. big endianness

Additionally, notes should be made about potentially ambiguous serialization behavior for types like Fw::String and the various buffer types. It should be made clear that serializing a primitive type (int, float or char) behaves exactly as you'd expect.