288481aabd doc: clarify that testnet min-difficulty is not optional (Sjors Provoost)
Pull request description:
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:
e568c1dd13/src/validation.cpp (L4212-L4215)
ACKs for top commit:
fjahr:
ACK 288481aabd
kevkevinpal:
ACK [288481a](288481aabd)
Tree-SHA512: 17d426301f386fa5810cceedfdb20a3523ab3ac2f17257ca7a525edd869fa409b150eff4cc258b27adecd0ded1c18ff48a9998fc9caed2faa461e410d4c5a884
1d281daf86