Merge pull request #3121 from nspcc-dev/neotest-doc

neotest: improve doc based on #3120

pkg/neotest/doc.go

@@ -15,5 +15,11 @@ Usually it's used like this:
 Higher-order methods provided in Executor and ContractInvoker hide the details
 of transaction creation for the most part, but there are lower-level methods as
 well that can be used for specific tasks.
+
+It's recommended to have a separate folder/package for tests, because having
+them in the same package with the smart contract iself can lead to unxpected
+results if smart contract has any init() functions. If that's the case they
+will be compiled into the testing binary even when using package_test and their
+execution can affect tests. See https://github.com/nspcc-dev/neo-go/issues/3120 for details.
 */
 package neotest