Expand functions docs to use language-specific terminology

[jkodroff]

In the Functions docs, we use some terminology that is not necessarily applicable in all languages, e.g. Get functions are not statically attached to a type in Go - they're just free-standing functions that are part of the namespace. For Provider Functions, we might want to ensure that the types are specifically mentioned (e.g. Promise in TS, Task in Python, etc.)

In order to enhance the docs, we should ensure that the functions pages are idiomatic and technically accurate for all languages.

I did attempt to do this for invokes (Provider Functions), but I couldn't get the signatures to display on a single line due to the layout, so I abandoned the effort.