Add class documentation FunctionSegmentNotDefinedErrorunexpected and make end_time optional

[ijemmy]

Description of your changes

Fix @saragerion 's comments on PR #828

1. Add class documentation on the FunctionSegmentNotDefinedErrorunexpected class
2. Make end_time optional as it can be undefined sometimes.

How to verify this change

Run E2E test

Related issues, RFCs

N/A

PR status

Is this ready for review?: Yes
Is it a breaking change?: NO

Checklist

• My changes meet the tenets criteria
• I have performed a self-review of my own code
• I have commented my code where necessary, particularly in areas that should be flagged with a TODO, or hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding changes to the examples
• My changes generate no new warnings
• The code coverage hasn't decreased
• I have added tests that prove my change is effective and works
• New and existing unit tests pass locally and in Github Actions
• Any dependent changes have been merged and published in downstream module
• The PR title follows the conventional commit semantics

Breaking change checklist

• I have documented the migration process
• I have added, implemented necessary warnings (if it can live side by side)

---

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[ijemmy]

E2E tests: https://github.com/awslabs/aws-lambda-powertools-typescript/actions/runs/2302043877

[dreamorosi]

~@ijemmy I'm merging a bunch of dependabot PRs that came up after we merged the last PR. After I'm done (5 left) could you please rebase?

EDIT: not needed, discard this message

[saragerion]

Thanks for addressing these, minor improvements but happy to merge it as it is

[saragerion]

Nit

Suggested change

	* Thrown when the function segement (AWS::Lambda::Function) is not found in a trace.
	* Thrown when the function segment (AWS::Lambda::Function) is not found in a trace.

[saragerion]

Suggested change

	* X-Ray segments are process asynchronously. They may not be available even after
	* X-Ray segments are processed asynchronously. They may not be available even after