docs: fix EnableConfigDiscovery summary to accurately describe agent discovery behavior

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

Update the summary to clarify that:
- Agent discovery is gated by EnableConfigDiscovery
- Use SkipCustomInstructions to independently control instruction file loading

Fixes #1887

Author: smz202000

dotnet/src/Types.cs

@@ -2983,8 +2983,9 @@ protected SessionConfigBase(SessionConfigBase? other)
     /// <see cref="McpServers"/> and <see cref="SkillDirectories"/>, with explicit
     /// values taking precedence on name collision.
     /// <para>
-    /// Custom instruction files (<c>.github/copilot-instructions.md</c>, <c>AGENTS.md</c>, etc.)
-    /// are always loaded from the working directory regardless of this setting.
+    /// Agent discovery (e.g. <c>.github/agents/</c>) is only performed when this is
+    /// <see langword="true"/>. Use <see cref="SkipCustomInstructions"/> to control
+    /// loading of custom instruction files independently.
     /// </para>
     /// </summary>
     public bool? EnableConfigDiscovery { get; set; }