[BEAM-14347] Update docs to prefer generic registration functions

[damccorm]

Now that we've proven out the effectiveness of the generic registration functions in #17613 and the 2.40 release is upon us, we should update the docs to recommend that approach.

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[damccorm]

R: @lostluck

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[damccorm]

Thanks for the feedback, should be fixed. I think I wrote these before I introduced register.FunctionXxY, and forgot about that class of updates - thanks for calling it out!

[lostluck]

One tiny change, but otherwise LGTM.