Update size measurement doc (#9179)

Add the description of explicit platform, iOS, to the doc.

Author: granluo

scripts/health_metrics/README.md

@@ -48,6 +48,10 @@ which will run [cocoapods-size](https://github.com/google/cocoapods-size)
 to create a binary size report and either post it in PRs in presubmits or
 upload it to the Metrics Service.
 
+The cocoapods-size tool measures an iOS SDK size by comparing the [64 bit ARM](https://github.com/google/cocoapods-size/blob/master/size_build_configuration.json#L5) slice
+before and after installing the pod to an [iOS skeleton app](https://github.com/google/cocoapods-size/tree/master/sizetestproject).
+More about CocoaPods Size measurement could be found [here](https://github.com/google/cocoapods-size#cocoapods-size-measurement).
+
 ### Replicate a binary size measurement for a released SDK
 
 To replicate an SDK binary size measurement locally, we can run the following
@@ -58,8 +62,6 @@ cd cocoapods-size
 python3 measure_cocoapod_size.py --cocoapods FirebaseABTesting
 ```
 
-More about CocoaPods Size measurement could be found [here](https://github.com/google/cocoapods-size#cocoapods-size-measurement)
-
 ### Environment Setup
 
 The `binary_size_metrics` on GitHub Actions [workflow](https://github.com/firebase/firebase-ios-sdk/blob/master/.github/workflows/health-metrics-presubmit.yml)