chore: update goma documentation (electron#21761)

Author: MarshallOfSound

docs/development/goma.md

@@ -9,9 +9,9 @@ on authentication.
 
 ## Enabling Goma
 
-Currently Electron Goma supports both Windows and Linux, we may add macOS
-support at some point in the future.  If you are on a supported platform
-you can enable goma by importing the `goma.gn` config file when using `gn`.
+Currently Electron Goma supports Windows, Linux, and macOS.  If you are
+on a supported platform you can enable goma by importing the `goma.gn` config
+file when using `gn`.
 
 ```bash
 gn gen out/Testing --args="import(\"//electron/build/args/testing.gn\") import(\"//electron/build/args/goma.gn\")"
@@ -39,9 +39,11 @@ goma_ctl.py ensure_start
 ## Building with Goma
 
 When you are using Goma you can run `ninja` with a substantially higher `j`
-value than would normally be supported by your machine.  Please do not set
-a value higher than **300**, we monitor the goma system and users found to
-be abusing it with unreasonable concurrency will be de-activated.
+value than would normally be supported by your machine.
+
+Please do not set a value higher than **300** on Windows or Linux and
+**80** on macOS, we monitor the goma system and users found to be abusing
+it with unreasonable concurrency will be de-activated.
 
 ```bash
 ninja -C out/Testing electron -j 200