Entries in servobuild.example file

Entries in the servobuild.example file that are not defaults were
commented out, defaults were mentioned in comments

servobuild.example

@@ -17,29 +17,33 @@ cargo-home-dir = "./.cargo"
 # rust-root is specified, will make sure that rust-root is in the path
 # when building. Similarly for Cargo. This takes care of PATH as well as
 # [DY]LD_LIBRARY_PATH.
+# rust-root and cargo-root default to <servo-repo>/
 system-rust = false
-rust-root = "/path/to/rust"
+#rust-root = "/path/to/rust"
 system-cargo = false
-cargo-root = "/path/to/cargo"
+#cargo-root = "/path/to/cargo"
 
 [build]
 # Set "mode = dev" or use `mach build --dev` to build the project with warning.
 # or Set "mode = release" or use `mach build --release` for optimized build.
-mode = "dev"
+# Defaults to prompting before building
+#mode = "dev"
 # Set "android = true" or use `mach build --android` to build the Android app.
 android = false
 # Set "debug-mozjs" or use `mach build --debug-mozjs` to build a debug spidermonkey.
 debug-mozjs = false
 
 # Android information
 [android]
-sdk = "/opt/android-sdk"
-ndk = "/opt/android-ndk"
-toolchain = "/opt/android-toolchain"
+# Defaults to the value of $ANDROID_SDK, $ANDROID_NDK, $ANDROID_TOOLCHAIN respectively
+#sdk = "/opt/android-sdk"
+#ndk = "/opt/android-ndk"
+#toolchain = "/opt/android-toolchain"
 
 # Gonk information
 # Please fill the ndk/toolchain for android too
 [gonk]
 # Path to B2G repo and build
-b2g = "/opt/B2G"
-product = "flame"
+# Defaults to the value of $GONKDIR
+#b2g = "/opt/B2G"
+product = "flame"