docs: add missing explanation for [angle|dawn]_enable_vulkan_validation_layers = false (#34216)

build/args/all.gn

@@ -21,6 +21,9 @@ proprietary_codecs = true
 ffmpeg_branding = "Chrome"
 
 enable_basic_printing = true
+
+# Removes DLLs from the build, which are only meant to be used for Chromium development.
+# See https://github.com/electron/electron/pull/17985
 angle_enable_vulkan_validation_layers = false
 dawn_enable_vulkan_validation_layers = false