gh-115684: Clarify datetime `replace` documentation (#116519)

* Clarify datetime `replace` documentation

In #115684, HopedForLuck noted that `datetime.date.replace()`
documentation was confusing because it looked like it would be changing
immutable objects.

This documentation change specifies that the `replace()` methods in
`datetime` return new objects. This uses similar wording to the
documentation for `datetime.combine()`, which specifies that a new
datetime is returned. This is also similar to wording for
`string.replace()`, except `string.replace()` emphasizes that a "copy"
is returned.

Resolves #115684.

* Include reviewer comments

Thanks Privat33r-dev for the comments!

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Co-authored-by: Paul Ganssle <1377457+pganssle@users.noreply.github.com>