docs: fix EnableConfigDiscovery summary to accurately describe agent discovery behavior

[smz202000]

Summary

Fixes #1887

The previous documentation for EnableConfigDiscovery incorrectly stated that custom instruction files are "always loaded from the working directory regardless of this setting." In practice, agent files (e.g. .github/agents/) are only discovered when EnableConfigDiscovery is true.

Changes

Updated the XML doc comment on SessionConfig.EnableConfigDiscovery in dotnet/src/Types.cs to:

1. Clarify that agent discovery is gated by EnableConfigDiscovery
2. Reference SkipCustomInstructions as the independent control for instruction file loading

Before:

/// <para>
/// Custom instruction files (.github/copilot-instructions.md, AGENTS.md, etc.)
/// are always loaded from the working directory regardless of this setting.
/// </para>

After:

/// <para>
/// Agent discovery (e.g. .github/agents/) is only performed when this is
/// true. Use SkipCustomInstructions to control
/// loading of custom instruction files independently.
/// </para>