[DO] Clarifying getAlarm behaviour if called while alarm is running

[Oxyjun]

Summary

Previously, the Alarms API (Workers Binding API) did not comment on the behaviour of getAlarm if an alarm was already running.

This PR adds in that missing detail, specifically, that:

• If getAlarm is called while an alarm is running, it will return null, unless setAlarm has been called since the alarm started running.
• This also implies (though not stated in the doc as it's redundant) that if you call getAlarm within an alarm, it will return null unless setAlarm has been called first inside the alarm.

Screenshots (optional)

Documentation checklist

• The documentation style guide has been adhered to.
• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.
• Files which have changed name or location have been allocated redirects.

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[github-actions]

Preview URL: https://0ddd200f.preview.developers.cloudflare.com
Preview Branch URL: https://jun-do-alarm-guidance.preview.developers.cloudflare.com

Files with changes (up to 15)

Original Link	Updated Link
https://developers.cloudflare.com/durable-objects/api/alarms/	https://jun-do-alarm-guidance.preview.developers.cloudflare.com/durable-objects/api/alarms/