Updated X-Ray Lambda function segment section

[willarmiros]

Description of changes:
Corrected the following items:

1. Include HTTP requests in "downstream calls" subsegments that are available
2. Removes the claim that you can add annotations to the function segment; this is not possible
3. Removes the claim that you need to use the X-Ray SDK to emit segments.

I also added an explanation of the behavior X-Ray exhibits when customers try to create custom segments in Lambda function code. It is a very common question (e.g. here and here and here). Happy to link X-Ray SDK docs or make other changes where necessary.

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