Update Doc Blocks to Discourage Use of Unix Timestamps

[oleibman]

This was suggested by issue #2347. Unix timestamps have clear disadvantages compared with the alternate methods of supplying date and time to PhpSpreadsheet - DateTime objects, Excel date time fields, and strings. In particular, Unix timestamp is not Y2038-safe on a 32-bit system, and it reflects the time in UTC, which may come as a surprise to the end-user (as it did in the cited issue). The alternate methods do not come with such baggage. This change updates some doc blocks to note that Unix timestamps are discoburage (N.B. - not deprecated). No executable code is changed.

This is:

- [ ] a bugfix
- [ ] a new feature
- [x] doc block updates

Checklist:

• Changes are covered by unit tests
• Code style is respected
• Commit message explains why the change is made (see https://github.com/erlang/otp/wiki/Writing-good-commit-messages)
• CHANGELOG.md contains a short summary of the change
• Documentation is updated as necessary

Why this change is needed?