Improve documentation

Author: AliSoftware

lib/fastlane/plugin/wpmreleasetoolkit/actions/ios/ios_send_app_size_metrics.rb

@@ -50,24 +50,39 @@ def self.description
       end
 
       def self.details
-        'Send iOS app size metrics to our metrics server'
+        <<~DETAILS
+          Send iOS app size metrics to our metrics server.
+
+          In order to get xcode generate the `app-thinning.plist` file (during `gym` and the export of the `.xcarchive`), you need to:
+            (1) Use either of `ad-hoc`, `enterprise` or `development` export method,
+            (2) Provide `thinning: '<thin-for-all-variants>'` as part of your `export_options` of `gym` (or in your `options.plist` file if you use raw `xcodebuild`)
+          See https://help.apple.com/xcode/mac/11.0/index.html#/devde46df08a
+
+          For builds exported with the `app-store` method, `xcodebuild` won't generate an `app-thinning.plist` file and you will only be able
+          to get the file size as a metric, but won't get the per-device, broken-down install and download sizes for each thinned variant.
+
+          See https://github.com/Automattic/apps-metrics for the API contract expected by the Metrics server you will send those metrics to.
+
+          Tip: If you provide a `file://` URL for the `api_base_url`, the action will write the payload on disk at the specified path
+          instead of sending the data to a endpoint over network. This can be very useful to inspect the data sent and debug without requiring a
+        DETAILS
       end
 
       def self.available_options
         [
           FastlaneCore::ConfigItem.new(
             key: :api_base_url,
             env_name: 'FL_IOS_SEND_APP_SIZE_METRICS_API_BASE_URL',
-            description: 'The endpoint API URL to publish metrics to',
+            description: 'The endpoint API URL to publish metrics to. (Note: you can also point to a `file://` URL to write the payload to a file instead)',
             type: String,
             optional: false
           ),
           FastlaneCore::ConfigItem.new(
             key: :api_token,
             env_name: 'FL_IOS_SEND_APP_SIZE_METRICS_API_TOKEN',
-            description: 'The bearer token to call to API',
+            description: 'The bearer token to call the API. Required unless `api_base_url` is a `file://` URL',
             type: String,
-            optional: true # Required unless `api_base_url` is a `file://` URL
+            optional: true
           ),
           FastlaneCore::ConfigItem.new(
             key: :app_name,