Fill in state change & migration details for FIP-0097 (Transient Storage)

[rvagg]

This one's for @snissn:

Now that FIP-0097 has been fully fleshed out in implementation and the migration is mostly complete, we should circle back to the FIP text to put in the details about the state changes and migration (git grep -i ^#.*migration to see previous FIPs that have such documentation):

• Addition of optional transient_data property in the evm state
• Document the structure of the TransientData object and the fact that it also points to a KAMT that holds the actual data
• Document a simple migration of adding this field (and making it null) to all existing evm actors

[BigLep]

2025-01-29 conversation: this will be a PR against the FIP repo. It's just adding detail.