Polish `merge/beacon-chain.md` with mostly non-substantive changes.
**Non-substantive changes**
* rename `MAX_EXECUTION_TRANSACTIONS` to `MAX_TRANSACTIONS_PER_PAYLOAD`
- rename "execution transaction" to just "transaction" as per discussion with Danny
* rename `compute_time_at_slot` to `compute_timestamp_at_slot`
- the function returns a Unix timestamp
- "timestamp" matches `execution_payload.timestamp`
* be explicit about `ExecutionEngine.execution_state` for clarity
* rename `ExecutionPayload.number` to `ExecutionPayload.block_number`
- more specific ("number" is pretty vague)
- consistent with `ExecutionPayload.block_hash`
* rename `new_block` to `on_payload`
- the `on_` prefix is consistent with other event handlers (e.g. see `on_tick`, `on_block`, `on_attestation` [here](https://github.com/ethereum/eth2.0-specs/blob/dev/specs/phase0/fork-choice.md#handlers))
- the `_payload` suffix is more to the point given the function accepts an `execution_payload`
- avoids conflict with `on_block` which is already used in the fork choice
* rework the table of contents for consistency
* order `is_execution_enabled` after `is_transition_completed` and `is_transition_block`
- `is_execution_enabled` refers to `is_transition_completed` and `is_transition_block`
* rename "transition" to "merge"
- "transition" is a bit vague—we will have other transitions at future hard forks
- there is no need for two words to refer to the same concept
* add a bunch of inline comments, e.g. in `process_execution_payload`
* make the `process_execution_payload` signature consistent with the other `process_` functions in `process_block` which take as arguments `state` and `block.body`
* remove `TRANSITION_TOTAL_DIFFICULTY`
- to be put in `merge/fork-choice.md` where it is used
* various misc cleanups
**Substantive changes**
* reorder `ExecutionPayload` fields
- for consistency with yellow paper and Eth1
- same for `ExecutionPayloadHeader`
- added comments separating out the execution block header fields from the extra fields (cosmetic)
