Improve documentation on cross contract calls with ink! contracts

[ltfschoen]

This is just a suggestion. I was referred in this message https://substrate.stackexchange.com/a/8723/83 to code here https://github.com/hyperledger/solang/blob/main/integration/substrate/ink/caller/lib.rs#LL22C1-L39C10 that uses invoke.

But personally I found that the error messages were not helpful and instead I used try_invoke and got it to work here in my toy project that is based on the basic-contract-caller example https://github.com/ltfschoen/InkTemplate/pull/16/files#diff-ba49fae9563c10308f352432d1f8c64a39b7e30cdf751bf0a3311ab0b3f2aaf7R166

So I'd like to suggest possibly adopting the same in Solang

[xermicus]

Well, I'd say for this integration test, it is not terribly important. We're only interested in whether the call goes through and works fine.

However, our documentation, for example in docs/targets/substrate.rst, deserves a new section containing concrete examples (possibly slightly more involved than this integration test) showing how to call ink! from Solidity and vice versa. If this is something you'd be interested in, we'd definitively welcome a PR!