C++: mention DD_APM_RECEIVER_SOCKET earlier in the setup doc

[dgoffredo]

What does this PR do?

Add a sentence to the C++ tracing setup guide about the Datadog Agent's "listen on a unix domain socket" feature.

Motivation

There's an initiative to get all tracing teams (programming languages) on the same page with respect to the use of unix domain sockets. Part of this is consistent customer documentation.

Preview

https://docs-staging.datadoghq.com/david.goffredo/c++-unix-domain-sockets/tracing/setup_overview/setup/cpp

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Reviewer checklist

• [ ] Review the changed files.
• [ ] Review the URLs listed in the Preview section.
• [ ] Check images for PII
• [ ] Review any mentions of "Contact Datadog support" for internal support documentation.