docs(routing): add error handling section

docs/1.guide/0.index.md

@@ -72,7 +72,7 @@ The `routes/` directory contains your application handlers. You can create subdi
 
 ### `api/`
 
-The `api/` directory is similar to `routes/` with only difference that routes inside it will be prefixed with `/api/` for convenience.
+The `api/` directory is similar to `routes/` with the main difference that routes inside it will be prefixed with `/api/` for convenience.
 
 :read-more{to="/guide/routing"}
 

docs/1.guide/2.routing.md

@@ -50,6 +50,11 @@ Some providers like Vercel use a top-level `api/` directory as a feature, theref
 You will have to use `routes/api/`.
 ::
 
+#### Differences between the two directories
+The `api/` directory is mainly for convenience, routes inside it will be prefixed with `/api/`.
+
+A notable difference between the two directories is how errors are handled by default: in `routes/` they are sent back with with a `Content-Type` of `text/html`, while in `api/` with `application/json`. This behaviour is overridden by some request properties (eg.: `Accept` or `User-Agent` headers).
+
 ### Simple routes
 
 First, create a file in `routes/` or `api/` directory. The filename will be the route path.