docs: ensure that it's clear that num signups account for empty leaf

[ctrlc03]

Description

Describe how to get the actual number of "real" users signed up for a maci poll.

Related issue(s)

fix #962

Confirmation

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Name	Status	Preview	Updated (UTC)
maci-website	✅ Ready (Inspect)	Visit Preview	Jun 17, 2024 7:14am

[ctrlc03]

thanks! should we just add this info to the latest version doc? or also update docs of other versions? (since this issue exists in every version?)

good call, added to v1.2 docs too