doc: add file validation documentation

doc.go

@@ -587,6 +587,14 @@ does any email provider accept all posibilities.
 
 	Usage: email
 
+File path
+
+This validates that a string value contains a valid file path and that 
+the file exists on the machine. 
+This is done using os.Stat, which is a platform independent function. 
+
+	Usage: file
+	
 URL String
 
 This validates that a string value contains a valid url