[apm]: Document sampling.tail.discard_on_write_failure config

[isaacaflores2]

Description

Adding description for new config that will be available on v8.19+. The description matches the one provided in the v9.1.0 docs: elastic/docs-content#1453

Documentation sets edited in this PR

Check all that apply.

• Stateful (docs/en/observability/*)
• Serverless (docs/en/serverless/*)
• Integrations Developer Guide (docs/en/integrations/*)
• None of the above

Related issue

Closes elastic/apm-server#15330

Checklist

• Product/Engineering Review
• Writer Review

Follow-up tasks

Select one.

• This PR does not need to be ported to another doc set because:
  • The concepts in this PR only apply to one doc set (serverless or stateful)
  • The PR contains edits to both doc sets (serverless and stateful)
• This PR needs to be ported to another doc set:
  • Port to stateful docs: <link to PR or tracking issue>
  • Port to serverless docs: <link to PR or tracking issue>

Testing

I kept getting the below error building the docs locally when running ../docs/build_docs --doc docs/en/observability/apm/configure/sampling.asciidoc --resource ../apm-server --resource ../ingest-docs/docs --open

INFO:build_docs:/docs_build/resources/asciidoctor/lib/chunker/extension.rb:102:in `page_containing': undefined method `context' for nil:NilClass (NoMethodError)

I validated with asciidoc plugin instead:

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[isaacaflores2]

[For reviewers] Note sure which Follow Up Tasks option to select and I am not sure if the backport-8.19 label is correct. Also please let me know if this PR should NOT be merged to the 8.X branch

[vishaangelova]

Left one tiny suggestion to match the docs-content PR, but otherwise LGTM