Edit APM docs

[dedemorton]

The APM documentation needs a thorough technical edit to fix duplication and clean up the wording.

Content that requires extra attention:

• Tuning topics - Addressed in Docs: WIP V2/6.5 additions and changes #1530, docs: WIP tuning and config updates #1576
• Content (added in Restructure docs for RUM agent and ES related parts. #1088) - Addressed in Docs: WIP V2/6.5 additions and changes #1530, docs: WIP tuning and config updates #1576
• Performance tuning docs (added in doc: add performance tuning doc apm-agent-nodejs#419) Addressed in Fix documentation spelling and grammatical errors apm-agent-nodejs#635, docs: api-v2 cleanup apm-agent-nodejs#665