Clarify descriptions for Cron API functions

[johnbillion]

The $timestamp parameter for the various WP-Cron functions expects a Unix timestamp (ie. UTC time), not a timestamp but this isn't made super clear.

In addition, the general inline docs for the cron functions needs some improvement.

[johnbillion]

37769.patch​ does the following:

• Standardises terminology to use 'event' instead of 'hook' or 'cron job'.
• Clarifies that $timestamp parameters expect a Unix timestamp.
• Documents the properties of the $event parameter of the schedule_event filter.
• Other misc improvements.

[wonderboymusic]

In 38357:

Cron: clarify descriptions for Cron API functions.

Props johnbillion.
Fixes #37769.