Merge #10682: Trivial: Move the AreInputsStandard documentation next to its implementation

Summary:
ca67ddf0b Move the AreInputsStandard documentation next to its implementation (esneider)

Pull request description:

The documentation (and rationale) for `AreInputsStandard` somehow got separated from its implementation, and creates a bit of confusion: it's in the middle of the file, next to the implementation of `IsStandard`, which actually checks the "standardness" of outputs, not inputs.

(this backport also restores the original formatting)

Backport of core PR10682

Test Plan: make check

Reviewers: #bitcoin_abc, deadalnix, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D3754