clarify initial premise of issue alerts

[souredoutlook]

Some users miss this point and expect issue alerts to be constantly scanning. It is reiterated further down after a screenshot though.

Legal Boilerplate

Look, I get it. The entity doing business as "Sentry" was incorporated in the State of Delaware in 2015 as Functional Software, Inc. and is gonna need some rights from me in order to utilize my contributions in this here PR. So here's the deal: I retain all rights, title and interest in and to my contributions, and by keeping this boilerplate intact I confirm that Sentry can use, modify, copy, and redistribute my contributions, under Sentry's choice of terms.

[vercel[bot]]

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Name	Status	Preview	Updated
sentry-docs	✅ Ready (Inspect)	Visit Preview	Sep 12, 2022 at 2:05PM (UTC)

[github-actions[bot]]

This pull request has gone three weeks without activity. In another week, I will close it.

But! If you comment or otherwise update it, I will reset the clock, and if you label it Status: Backlog or Status: In Progress, I will leave it alone ... forever!

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"A weed is but an unloved flower." ― Ella Wheeler Wilcox 🥀