docs: add note about compile-time interface checks

[lin-snow]

@chai2010

What

• Added explanation to the interface assertion example in Go docs.
• Clarifies that var _ io.Writer = (*bytes.Buffer)(nil) enforces compile-time type checks.

Why

• Helps readers understand the practical use of this idiom.
• Ensures that future changes triggering interface mismatches are caught at compile time.

How to verify

• Rendered documentation locally.
• Confirmed that the code snippet compiles as expected.