docs(RegisterConnConfig): explain alternative usage

[AldoFusterTurpin]

Context
The fact that RegisterConnConfig "holds" an internal map that can grow without bound is something that could be stated explicitly.

PR changes
I believe it would be a good idea to explain an alternative for clients with many short-lived connections to many DBs, to avoid memory leaks. Being a bit more explicit and vocal in the comments about how that works will help clients to choose if they really want to use RegisterConnConfig or the alternative shared.

I placed the comment both in

• RegisterConnConfig() (I believe this would be the perfect place as when you use a library you just check the docs from your editor and sometimes don't browse pkg.go.dev),
• the top of the file in case people is browsing the documentation in https://pkg.go.dev/ so they can read through.

WDYT ?

Thank you!

[AldoFusterTurpin]

@jackc (Sorry for the ping) I believe this is pretty straightforward and can help new people using the module 🙂