added note about laziness of operations to statistical functions

[corinnebosley]

Only two functions to clarify here. This is a small one to start us off.

[corinnebosley]

@valeriupredoi @ledm How's this format for you?

[ledm]

Good one, thanks!

[corinnebosley]

@pp-mo @bjlittle @lbdreyer I've got a whole load of these PRs coming up, if somebody could spare the time to check them over as they come up I'd be really grateful.

[valeriupredoi]

music to my ears 😁 cheers @corinnebosley 🍺

[lbdreyer]

I may be missing some context here as to the purpose of this PR, but this line doesn't seem necessary to add.

I'm assuming your intention is to clarify which functions are lazy for the user. If that is the case, it is surely not necessary to add this to a private function, which won't be included in the documentation?

[rcomer]

I guess this note would be useful for the next developer who looks at the code, wanting to make the public function lazy?

[bjlittle]

@corinnebosley Thanks.