docs: the sentences the rewrite reviews left as follow-up notes (closes #723)

Each sentence named on #723 now
matches the code on `next`:

- the Firefox load steps are numbered;
- the AddToken refusal lists name a token that fakes a known symbol;
- the worker's in-memory state is listed in full;
- `script/bootstrap` falls back to `npm install -g` for yarn;
- the Send line under a `null` ETH balance holds with ETH selected;
- the Home and Send lines there name `SepoliaETH` on Sepolia, and the flash
  message "Max" shows;
- the well-known symbols are given per network;
- a network switch empties the fraud contract list;
- the address scan's two misses are listed;
- a new token's line depends on "Show tracked tokens with zero balance";
- wallet drainer is defined at first use.

Model: fable-5-1

README.md

@@ -1235,9 +1235,11 @@ holds for a new address, and for every address after a network switch. While it
 is `null`:
 
 - the balance lists show "quantity unknown";
-- Home's ETH balance reads `unknown (ETH)`;
-- with ETH selected, Send reads `Current balance: unknown (ETH)`, and "Max"
-  fills in nothing;
+- Home's ETH balance reads `unknown (ETH)` on mainnet and `unknown (SepoliaETH)`
+  on Sepolia;
+- with ETH selected, Send reads `Current balance: unknown (ETH)` on mainnet and
+  `Current balance: unknown (SepoliaETH)` on Sepolia, and "Max" fills in nothing
+  and shows the flash message "This token's balance is unknown.";
 - "Send" is not refused for a zero balance;
 - the address has no total;
 - DeleteAddress shows its warning.