Add "chain-specific notes" for more chains

[brson]

As more people notice realtps and more chains look to be included, we're probably going to want to be as precise as we can about what we are counting.

These chains already differ notably from the EVM model:

• NEAR - has multiple shards
• Stellar - there is some nuance noted in the client implemenation about what is counted as a transaction, and I have not looked at it closely