Specify that hidden service version numbers are positive integers

Clarification for #15010. This specifies that both the hidden service
descriptors and INTRODUCE cell versions must be positive ints. I tried to find
a more succinct example in the dir-spec (thought there might be a POSITIVE_INT
type or something like that) but no luck.

rend-spec.txt

@@ -277,7 +277,8 @@
 
        [Exactly once]
 
-       The version number of this descriptor's format.  In this case: 2.
+       The version number of this descriptor's format. Version numbers are a
+       positive integer.
 
      "permanent-key" NL a public key in PEM format
 
@@ -309,7 +310,7 @@
 
        A comma-separated list of recognized and permitted version numbers
        for use in INTRODUCE cells; these versions are described in section
-       1.8 below.
+       1.8 below. Version numbers are positive integers.
 
      "introduction-points" NL encrypted-string