Improve documentation on \Drupal\Component\Datetime\DateTimePlus::setDefaultDateTime()

Problem/Motivation

In #2993165: Date Only field shows incorrect default value when UTC date is different than user's date we discovered that the documentation on \Drupal\Component\Datetime\DateTimePlus::setDefaultDateTime() should be improved.

This task was created to do this.

I think we should reconsider the documentation of \Drupal\Component\Datetime\DateTimePlus::setDefaultDateTime() - on its own this method leads to this problem. It's called from a few places in core but this is the only place where the timezone on the date might not be UTC. The other runtime place is \Drupal\datetime\DateTimeComputed::getValue() - fwiw I'm not sure if implementation is correct. Dates and timezone stuff is hard to reason about. But this should be in a follow-up to improve the docs on setDefaultDateTime

Thus spoketh @alexpott in #2993165-58: Date Only field shows incorrect default value when UTC date is different than user's date

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

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