Improve API YAML info.description documentation clarity

[benhepworth]

Summary

The info.description field and inline descriptions throughout code/API_definitions/session-insights.yaml need significant improvements to help API consumers understand the API correctly.

Changes Required

• Change "network operator" → "API provider" throughout
• Clarify the three session identifiers and their relationships:
  • sessionId (path parameter, schema defined ~line 312)
  • id (property of SessionBase object, ~line 354)
  • applicationSessionId (property of SessionBase object, ~line 358)
  • These have little or no description — document the purpose and relationship of each
• Describe cardinality of object relationships, e.g.:
  • 1 device → multiple applications?
  • 1 application → 1 application-profile?
  • 1 application → multiple sessions?
• Expand the Authorization and Authentication section to explain how the device is linked to the API call
• Expand the error code section with similar context
• Standardize terminology: use either "KPI" or "KPI metrics" consistently (not both)
• Change "existing Application Profile" → "selected Application Profile" (obtained via GET application-profiles)
• Remove the RCA abbreviation — use plain language such as "event indicating the problem cause"
• Expand the WMM abbreviation (line 33) — no unexplained abbreviations
• Overall: elaborate descriptions to help API users understand the intent and correct usage

References

• Raised during review of PR r1.1 M3 release #55

[mohdfarhanakram]

@benhepworth I will take this one - already working through the related fixes in this area (#67 is up in #75).