Emphasize keytocard danger and fix inconsistency.

This commit applies a few editorial cleanups to the document:

  1. `keytocard` operations now contained emphasized warnings to convey
     that these operations are destructive.  I unknowingly made this
     mistake a few years ago and only learned of it recently.  For that
     reason, we should go out of our way on user's behalf with due
     diligence warnings.

  2. `$KEYID` was not uniformly used throughout the document in various
     command line input literals.  This is now fixed.

  3. `YubiKey` was often represented as `Yubikey` and other
     inconsistent forms throughout the document.  This is now fixed,
     except in cases of URL, command output, etc.

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