[IF-INTERLAY2-06], [IF-INTERLAY2-07] Minor suggestions on Issue, Redeem, and Replace specs

[shonfeder]

This PR suggests a few very minor fixes/alterations surfaced from @informalsystems' audit of interlay/btc-parachain at 75fd3e2

The only suggestion that has any substance is that in 655f606, please see that commit for details.

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[shonfeder]

Fixed. Thanks!