Add diargam explaining how ThrottlingRequest works

https://asciiflow.com/#/share/eJzFV71u2zAQfhWCkx0EDdqhRbUEGbpmCLoE0BBCpiPBEplSFBwhCFoEHTt0MNw%2BRKeMRZ9GT9KjrB8ykhj9pRUIWTR1d9%2Fdfcej7jAjEcUOS8LwGIckpQI7%2BM7Fty523r9%2Be%2BziFJ7evFNPkt5KmLj45AT1vrLdH9dlXRIffcGlDAN2fUE%2FJTSWBwm4b7nYULFYWoSHm4PVoaMENELUMgqM41Xsf8NdFDELYhRTJk20MaVFENGKe0kELxAZcHZ4a%2BL1uXZg9jATtkIejxTgGK25QBEX1Eit3QVztdQaMFAV5RFAMHy%2BRYEconXfppV4MiFhmOaRjo3157T2CQl6YfKNVIwKgkGiQu5tdKymbp%2FEFQcDujpV4j%2B%2BVnokL1ZbCe78Q196Z0Lng5Hhp3nuyvvkfBpWL1sodW53w0CRT44ich14R%2B3M7fKjq87m8cusNsNqsw712XA%2B1IEbRx0Tm2k%2FL5GAMeB%2FbWlLYONR%2B1pRGZqmOYKX1X2hBpjlxQY7FWx7ChQ0XCVY1qJTPaErVdGvEqZ%2BFsurSTGVPnSmBpjZPG0SYFIi7Zp7Ua5JUAtxTblubKNDZfUvp4lMBCttaFytUmaTjZNQlrJ5y%2Fapt4GeqjJerf1%2FPyHuj9qPkb4medo5oE8%2FCLG4oFKkZ2sJ51O2XD4vScOYTrHZTiGr7EwBtlnRcRA4oQGdREBjOF2dDpB8EURd3bhVss84r4zX12W7RuulJzfbfct2X0aN74Ns7nWbD%2Fl%2FT1r%2BQ7e%2B6fWobrtuX35ZPf15QDbCLOMyWKdlk5U%2B0b%2BPtOA8VhYUgdNy81OqtKNqXfKICgHNW4M06Kg6whN1ivbJzQ1ldKXTKOv%2B1p2etRHf3fge3%2F8FrCjqXw%3D%3D)

src/adaptors/throttle/request.rs

@@ -90,6 +90,53 @@ where
     }
 }
 
+// This diagram explains how `ThrottlingRequest` works/what `send` does
+//
+//                                          │
+//                      ThrottlingRequest   │   worker()
+//                                          │
+//                      ┌───────────────┐   │  ┌────────────────────────┐
+//  ┌──────────────────►│request is sent│   │  │see worker documentation│
+//  │                   └───────┬───────┘   │  │and comments for more   │
+//  │                           │           │  │information on how it   │
+//  │                           ▼           │  │actually works          │
+//  │                      ┌─────────┐      │  └────────────────────────┘
+//  │ ┌────────────────┐   │send lock│      │
+//  │ │has worker died?│◄──┤to worker├─────►:───────────┐
+//  │ └─┬─────────────┬┘   └─────────┘      │           ▼
+//  │   │             │                     │  ┌──────────────────┐
+//  │   Y             └─N───────┐           │  │     *magic*      │
+//  │   │                       │           │  └────────┬─────────┘
+//  │   ▼                       ▼           │           │
+//  │ ┌───────────┐    ┌────────────────┐   │           ▼
+//  │ │send inner │    │wait for worker │   │  ┌─────────────────┐
+//  │ │request    │    │to allow sending│◄──:◄─┤ `lock.unlock()` │
+//  │ └───┬───────┘    │this request    │   │  └─────────────────┘
+//  │     │            └────────┬───────┘   │
+//  │     │                     │           │
+//  │     ▼                     ▼           │
+//  │    ┌──────┐  ┌────────────────────┐   │
+//  │    │return│  │send inner request  │   │
+//  │    │result│  │and check its result│   │
+//  │    └──────┘  └─┬─────────┬────────┘   │
+//  │     ▲    ▲     │         │            │
+//  │     │    │     │ Err(RetryAfter(n))   │
+//  │     │    │   else        │            │
+//  │     │    │     │         ▼            │
+//  │     │    └─────┘  ┌───────────────┐   │
+//  │     │             │are retries on?│   │
+//  │     │             └┬─────────────┬┘   │
+//  │     │              │             │    │
+//  │     └────────────N─┘             Y    │
+//  │                                  │    │  ┌──────────────────┐
+//  │                                  ▼    │  │     *magic*      │
+//  │                ┌──────────────────┐   │  └──────────────────┘
+// ┌┴────────────┐   │notify worker that│   │           ▲
+// │retry request│◄──┤RetryAfter error  ├──►:───────────┘
+// └─────────────┘   │has happened      │   │
+//                   └──────────────────┘   │
+//                                          │
+
 /// Actual implementation of the `ThrottlingSend` future
 async fn send<R>(
     mut request: ShareableRequest<R>,