Update Lambda Log Forwarder docs troubleshooting - timestamps

[mirzanurkic-dd]

What does this PR do?

Adds a section to the Datadog Lambda Forwarder documentation about valid timestamps, referencing the Logs timestamp remapper documentation

Motivation

A customer support ticket where this was the root cause for logs not appearing in Datadog: https://datadog.zendesk.com/agent/tickets/1385418

Testing Guidelines

Previewed the markdown locally, otherwise untested

Additional Notes

Types of changes

• Bug fix
• New feature
• Breaking change
• Misc (docs, refactoring, dependency upgrade, etc.)

Check all that apply

• This PR's description is comprehensive
• This PR contains breaking changes that are documented in the description
• This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• This PR impacts documentation, and it has been updated (or a ticket has been logged)
• This PR's changes are covered by the automated tests
• This PR collects user input/sensitive content into Datadog
• This PR passes the integration tests (ask a Datadog member to run the tests)
• This PR passes the unit tests
• This PR passes the installation tests (ask a Datadog member to run the tests)

[maycmlee]

Just a couple of suggestions!