Add package-level doc comments for go doc and gopls

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

retry/doc.go

@@ -0,0 +1,31 @@
+// Package retry provides configurable HTTP request retry as client middleware.
+//
+// The retry middleware wraps an http.RoundTripper and automatically retries
+// failed requests based on a configurable policy, with exponential backoff
+// and optional jitter.
+//
+// # Usage
+//
+//	mw := retry.Transport(
+//	    retry.WithMaxAttempts(3),
+//	    retry.WithBackoff(retry.ExponentialBackoff(100*time.Millisecond, 5*time.Second)),
+//	)
+//	transport := mw(http.DefaultTransport)
+//
+// # Retry-After
+//
+// The retry middleware respects the Retry-After response header. If a server
+// returns 429 or 503 with Retry-After, the delay from the header overrides
+// the backoff strategy.
+//
+// # Request bodies
+//
+// For requests with bodies to be retried, the request must have GetBody set.
+// Use httpx.NewJSONRequest or httpx.NewFormRequest which set GetBody
+// automatically.
+//
+// # Sentinel errors
+//
+// ErrRetryExhausted is returned when all attempts fail. The original error
+// is wrapped and accessible via errors.Unwrap.
+package retry