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>

[SteveSandersonMS]

Reviewed and extended this PR to apply one consistent EnableConfigDiscovery description across all 6 SDKs (dotnet/go/nodejs/python/rust/java), replacing per-language wording that varied and, in 4 languages, incorrectly claimed custom instruction files are always loaded regardless of this setting.

Validation performed:

• Built/typechecked dotnet (net8.0), rust (cargo build), nodejs (tsc --noEmit), python (ruff check) — all clean. Go/Java toolchains unavailable locally but changes are comment-only with balanced braces verified.
• Manually reproduced the underlying behavior against a live copilot CLI runtime: with EnableConfigDiscovery=false, .github/agents/*.agent.md is NOT discovered (agent.list() returns []); with true it is. Separately confirmed AGENTS.md content is loaded into the model's context even when EnableConfigDiscovery=false (model referenced the exact planted marker as a 'custom_instruction').
• Full CI matrix (58 checks) passed across all languages after approving the required workflow runs.