diff --git a/docs/api/protocol.md b/docs/api/protocol.md
index a076e3640daa..2734546516e3 100644
--- a/docs/api/protocol.md
+++ b/docs/api/protocol.md
@@ -37,6 +37,11 @@ Registers a custom protocol of `scheme`, the `handler` would be called with
 You need to return a request job in the `handler` to specify which type of
 response you would like to send.
 
+By default the scheme is treated like `http:`, which is parsed differently
+from protocols that follows "generic URI syntax" like `file:`, so you probably
+want to call `protocol.registerStandardSchemes` to make your scheme treated as
+standard scheme.
+
 ## protocol.unregisterProtocol(scheme, callback)
 
 * `scheme` String
@@ -48,7 +53,11 @@ Unregisters the custom protocol of `scheme`.
 
 * `value` Array
 
-`value` is an array of custom schemes to be registered to the standard.
+`value` is an array of custom schemes to be registered as standard schemes.
+
+A standard scheme adheres to what RFC 3986 calls
+[generic URI syntax](https://tools.ietf.org/html/rfc3986#section-3). This
+includes `file:` and `filesystem:`.
 
 ## protocol.isHandledProtocol(scheme, callback)