docs: README says why the wallet never clears the clipboard (closes #492)

The owner's ruling on #19 is
now in the README, under ExportPrivKey: copying the key leaves it on the
clipboard, because the clipboard is the user's, clearing it would go
against what they expect, and it could destroy something else they
copied since. ShowRecoveryPhrase copies the phrase the same way and
points back to it. Both describe the warning each password screen
shows on next, which does not mention the clipboard.

Model: opus-5-5

README.md

@@ -1510,6 +1510,14 @@ view would leave a wallet one click from deletion.
   route, including the Settings gear. A decrypt still running when the screen is
   left is discarded rather than written. The screen is not restorable, so
   reopening the popup lands on Home rather than back on the key.
+- **Clipboard**: tapping the key copies it to the clipboard, and the wallet
+  never clears the clipboard afterwards; leaving the screen wipes the key from
+  the page only. The clipboard is the user's, not the wallet's. Clearing it
+  would go against what the user expects, and by then they may have copied
+  something else vital that the clear would destroy. The user knows the key is
+  secret from the warning above the password input, which says that anyone with
+  it can access and transfer all funds from the address, and knows it is on the
+  clipboard because they copied it. From then on it is theirs to manage.
 
 #### AddressToken (`address-token`)
 
@@ -1840,6 +1848,9 @@ view would leave a wallet one click from deletion.
   gear. A decrypt still running when the screen is left is discarded rather than
   written. The screen is not restorable, so reopening the popup lands on Home
   rather than back on the phrase.
+- **Clipboard**: tapping the phrase copies it, and the wallet never clears the
+  clipboard afterwards, for the reasons given under ExportPrivKey; here the
+  warning box above the password input is what tells the user it is secret.
 
 #### DeleteWallet (`delete-wallet-confirm`)