[events] refine general event semantics documentation

[pellared]

Towards stabilization of https://github.com/open-telemetry/semantic-conventions/blob/main/docs/general/events.md

Fixes #1651

Changes

This PR refines the documentation for general event semantics to improve clarity and structure.

Documentation improvements

• Restructure event semantics section for better readability
• Consolidate event name definition with its uniquely identifying role
• Simplify cardinality and naming guidelines into single concise statement

Requirement level clarifications

• Attributes: Strengthen from MAY to SHOULD to emphasize their importance for providing event context
• Severity number: Strengthen from MAY to SHOULD to encourage consistent severity tracking
• Remove explicit "body MAY be present" statement (still implied in first bullet) given unclear use cases. Body SHOULD NOT be used.

Technical changes

• Consistent hyperlinking to all specification fields (event name, attributes, body, severity number)
• Align with RFC 2119 keyword usage patterns (use SHOULD instead of RECOMMENDED)
• Improve consistency with naming guidelines reference
• Enhanced external event compatibility guidance with explicit instrumentation SHOULD

Rationale

The changes improve developer experience by:

1. Making requirements clearer and easier to scan
2. Emphasizing best practices (using attributes and severity numbers)
3. Reducing redundancy while maintaining completeness
4. Better aligning with the structure used in other semantic convention documents
5. Providing more concise and actionable guidance for convention authors

[pellared]

I do not think this needs a changelog. Feel free to remove the label if you disagree.

[Copilot]

Pull Request Overview

This PR refines the documentation for OpenTelemetry Events to improve clarity and better align with semantic conventions standards. The changes reorganize and consolidate event requirements while maintaining technical accuracy.

• Consolidated event definition requirements into a clearer structure
• Improved the phrasing of event semantics to be more concise and consistent with RFC 2119 keywords
• Enhanced guidance for external event compatibility scenarios

[lmolkova]

didn't realize I had pending comments, posing now