correct docs for TerminalError (#53)

doc.go

@@ -157,7 +157,11 @@ that has a slice in it is not okay.
 
 Fallible injectors
 
-Fallible injectors are injectors that return a value of type TerminalError.
+Fallible injectors are special injectors that change the behavior of the injection
+chain if they return error.  Fallible injectors in the RUN set, that return error
+will terminate execution of the injection chain.
+
+A non-wrapper function that returns nject.TerminalError is a fallible injector.
 
 	func(input value(s)) (output values(s), TerminalError)
 

types.go

@@ -19,11 +19,8 @@ import (
 // and (like other return values) it must be consumed by an upstream handler
 // or the invoke function.
 //
-// Functions that return TerminalError do not count as having no outputs and
-// thus they are not treated as specially required.  The when TerminalError is
-// returned, if it is nil, then it provides a nil error to the rest of the
-// downard chain. If it returns non-nil, then it provides a non-nil error as a
-// returned value to the upwards chain.
+// Functions that return TerminalError do count as having no outputs and
+// thus they are treated as specially required if they're in the RUN set.
 //
 // Note: wrapper functions should not return TerminalError because such
 // a return value would not be automatically converted into a regular error.