docs: improve docs around driver init funcs

[amoeba]

We saw some feedback recently that some of the convention around driver shared library name and driver init func names could be clearer. This PR aims to at least improve our language around init func names.

I think what we basically want to say, informally, is:

All drivers should export two init functions, AdbcDriverInit and AdbcDriverFooInit. Drivers must always export AdbcDriverInit and drivers should also export AdbcDriverFooInit. It is also recommended that drivers support conditionally compiling AdbcDriverInit out in order to support static linking of multiple drivers into a single application. Hence our recommendation of exporting AdbcDriverFooInit.

[ianmcook]

Thanks for doing this.

I found this a bit hard to follow. Is there a way to make the language clearer and more imperative?

How about also explaining that, regardless of what file name(s) that you as the developer choose for your shared libraries, popular ADBC driver registries systems such as dbc-cdn.columnar.tech will always rename them in the form libadbc_driver_foo?