docs: Answer first-time questions in the user guide

[BenWestgate]

A tester asked several questions that the user guide doesn't answer. Add a short answer to each.

• Why the master fingerprint goes on the separate wallet record, not on the cards. A fingerprint on a card would let anyone holding k−1 shares test candidate secrets against it. Kept off the cards, k−1 shares reveal nothing about the secret, which is the perfect information-theoretic secrecy that Shamir sharing provides. Fresh shared sets already use random identifiers, so the cards carry no fingerprint bits (see "Identifier policy" in the API guide). A record stored apart from the cards is also independent evidence at restore time (Verify CLI recovery identity before wallet import #30).
• Why restore asks you to type the fingerprint. A mistaken correction, a card from another backup, or a typo in a hex seed each produce a valid wallet that the cards can't recover. Typing the fingerprint from the record makes the software compare all eight characters before Bitcoin Core is touched, and it can't be done without the record at hand. Without a record, there is an explicit visual path with a warning.
• How you know a string's length while typing it. Neither ms1 nor the header encodes the length. Valid Bitcoin master-seed lengths are 48, 54, 61, 67, 74 and 127 characters. The first string entered sets the length for the rest, and the length-specific cards from docs: Size recovery cards to the backup length #88 show where the string ends.
• Share indices and S. create with a threshold shows shares at random indices and never shows S. create 0 makes an unshared secret, and secret rebuilds it from shares. Each run makes a new seed and identifier.
• Letter case. A codex32 string is either all uppercase or all lowercase. Case doesn't change the seed or the fingerprint. correct: Say why the entry was invalid when suggesting a repair #85 adds this to correction messages.