Update README to Make Offline Storage Behavior Clear

[JacksonWeber]

This pull request updates the README.md documentation to clarify the behavior of offline storage and related configuration options in the Application Insights 3.X SDK and Azure Monitor OpenTelemetry. The changes provide more detailed information about hardcoded defaults for resend interval, maximum disk usage, and file retention for cached telemetry data.

Configuration documentation improvements:

• Added explicit documentation that the SDK uses hardcoded defaults for offline storage: resend interval is 60 seconds (1 minute), maxBytesOnDisk is 50 MB, and cached files are retained for up to 2 days before cleanup in the setUseDiskRetryCaching section.
• Updated the enableResendInterval and enableMaxBytesOnDisk configuration descriptions to specify that these values are hardcoded (not configurable): resend interval is 60 seconds (1 minute), maxBytesOnDisk is 50 MB, and cached files are cleaned up after 2 days. Also clarified that new telemetry is not cached once the disk limit is reached.

[rads-1996]

LGTM