doc: clarify that testnet min-difficulty is not optional #32095

When 20 minutes have gone by on testnet3 or testnet4, the next block MUST have difficulty 1. I've seen people be confused about this several times now in recent months. It doesn't help that the code comment is wrong. So fixing that.

The reason is that nBits must match exactly:

https://github.com/bitcoin/bitcoin/blob/e568c1dd134e0318c46113cb7dfc23b40e2829e8/src/validation.cpp#L4212-L4215

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/32095.

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