add note about pricing to identify docs

Changes

Someone said the docs don't make it clear that person profiles is charged separately (it's only on the pricing page). We should mention it, specifically because it's kind of a new concept in the industry.

Group analytics doesn't mention it - but it's also only charged if you are subscribed (a manual action someone has to take) AND it's a standard concept in analytics to pay for groups separately.

This PR adds a note to the identify and user properties docs saying we only recommend capturing them when necessary.

Checklist

[ ] Words are spelled using American English
[ ] Titles are in sentence case
[ ] Feature names are in sentence case too
[ ] Use relative URLs for internal links
[ ] If I moved a page, I added a redirect in vercel.json
[ ] Remove this template if you're not going to fill it out!

Article checklist

[ ] I've added (at least) 3-5 internal links to this new article
[ ] I've added keywords for this page to the rank tracker in Ahrefs
[ ] I've checked the preview build of the article
[ ] The date on the article is today's date
[ ] I've added this to the relevant "Tutorials and guides" docs page (if applicable)

PostHog / posthog.com