[DOC] Keep "function call"naming consistent

[bucanero]

As discussed on Zulip (@volovyk-s @BenKurrek @austinabell @bowenwang1996 ), we should keep our naming consistent across documentation.
Currently, we are using both function call and method call to talk about smart contract calls.

We have decided to stick with function call for consistency, and we need to update DevDocs to reflect that.

[bucanero]

Hello @BenKurrek, I saw your related issue here: #899 , but from Zulip's discussion we'll stick to function call.
I'm not sure if we need to update issue 899 too? 🤔