Make documentation of ThenFunc more concise.

chain.go

diff --git a/chain.go b/chain.go
index dac1f54..97b1c6d 100644
--- a/chain.go
+++ b/chain.go
@@ -60,24 +60,14 @@
 	return final
 }
 
-// Then chains the middleware and returns the final http.Handler.
-//     New(m1, m2, m3).ThenFunc(h)
-// is equivalent to:
-//     m1(m2(m3(http.HandlerFunc(h))))
-// When the request comes in, it will be passed to m1, then m2, then m3
-// and finally, the given handler
-// (assuming every middleware calls the following one).
+// ThenFunc works identically to Then, but takes
+// a HandlerFunc instead of a Handler.
 //
-// A chain can be safely reused by calling ThenFunc() several times.
-//     stdStack := alice.New(ratelimitHandler, csrfHandler)
-//     indexPipe = stdStack.ThenFunc(indexHandlerFunc)
-//     authPipe = stdStack.ThenFunc(authHandlerFunc)
-// Note that constructors are called on every call to Then()
-// and thus several instances of the same middleware will be created
-// when a chain is reused in this way.
-// For proper middleware, this should cause no problems.
+// The following two statements are equivalent:
+//     c.Then(http.HandlerFunc(fn))
+//     c.ThenFunc(fn)
 //
-// ThenFunc() treats nil as http.DefaultServeMux.
+// ThenFunc provides all the guarantees of Then.
 func (c Chain) ThenFunc(fn http.HandlerFunc) http.Handler {
 	return c.Then(http.HandlerFunc(fn))
 }