Mention things that x can't do

There are a few pattern in protocols and serialization formats that X can't emulate. I think they're worth noting somewhere:

- Structs that can be extended by adding new elements to the end
- Structs that contain a variable-length member at the end
- Structs with key-value pairs, where it's not necessary to annotate elements with indices
- Flags earlier in the data that indicate the presence or absence of elements later in the data
- Integer values in the data that indicate the length of later elements in the data

No file, test, or entry point is named. Start by locating the project's documentation and choose the appropriate place to explain these five serialization patterns that X cannot emulate. Done means all five limitations are documented clearly and accurately.

Written by the indexing model from the issue text.