Clarify interpretation of secondaryFiles on inputs.

Author: Peter Amstutz

v1.0/Process.yml

@@ -336,12 +336,14 @@ $graph:
         Only valid when `type: File` or is an array of `items: File`.
 
         Describes files that must be included alongside the primary file(s).
+        All listed secondary files must be present.  An implementation may
+        fail workflow execution if a secondary file does not exist.
 
         If the value is an expression, the value of `self` in the expression
         must be the primary input or output File to which this binding applies.
 
         If the value is a string, it specifies that the following pattern
-        should be applied to the primary file:
+        should be applied to the `location` of the primary file:
 
           1. If string begins with one or more caret `^` characters, for each
             caret, remove the last file extension from the path (the last