Clarify Rack middleware_names option (#1262)

This PR expands on the explanation of the `middleware_names` option for Rack.

This option sets the Rack resource name to the last executed Rack middleware, which show match the name of the middleware responsible for handling the request.

The existing description is a bit vague, and can be misinterpreted as enabling the tracing of all Rack middleware layers.

docs/GettingStarted.md

@@ -1257,7 +1257,7 @@ Where `options` is an optional `Hash` that accepts the following parameters:
 | `application` | Your Rack application. Required for `middleware_names`. | `nil` |
 | `distributed_tracing` | Enables [distributed tracing](#distributed-tracing) so that this service trace is connected with a trace of another service if tracing headers are received | `true` |
 | `headers` | Hash of HTTP request or response headers to add as tags to the `rack.request`. Accepts `request` and `response` keys with Array values e.g. `['Last-Modified']`. Adds `http.request.headers.*` and `http.response.headers.*` tags respectively. | `{ response: ['Content-Type', 'X-Request-ID'] }` |
-| `middleware_names` | Enable this if you want to use the middleware classes as the resource names for `rack` spans. Requires `application` option to use. | `false` |
+| `middleware_names` | Enable this if you want to use the last executed middleware class as the resource name for the `rack` span. If enabled alongside the `rails` instrumention, `rails` takes precedence by setting the `rack` resource name to the active `rails` controller when applicable. Requires `application` option to use. | `false` |
 | `quantize` | Hash containing options for quantization. May include `:query` or `:fragment`. | `{}` |
 | `quantize.query` | Hash containing options for query portion of URL quantization. May include `:show` or `:exclude`. See options below. Option must be nested inside the `quantize` option. | `{}` |
 | `quantize.query.show` | Defines which values should always be shown. Shows no values by default. May be an Array of strings, or `:all` to show all values. Option must be nested inside the `query` option. | `nil` |