Clearer validation and instruction on NWC strings

[huumn]

Most people try to put the same string in for send/receive and our validation messages suck.

The rizful founder says everyone that tries to attach to SN is confused.

[ekzyis]

Interesting, they even have a good guide.

This is somewhat related to #1497.

I guess the message that both can't be the same sucks the most since it doesn't give you any information about what you should do instead.

[huumn]

Yeah, most people using NWC are only using one string and don't understand why we need two. Plus, other apps are storing send perms on their servers in plaintext. 😅

We probably need to go above an beyond explaining the reasoning and what permissions, exactly, each string requires.