AIP-162: Clarify UUID4 (aip-dev#874)

* AIP-162 - Clarified the term UUID4

UUID is a common term but the usage of UUID4 is not usual and
requires user to search for definitions. To simplify reading
of the AIP and provide better reference this change provides
link to wikipedia article about UUID Verions 4.

aip/general/0162.md

@@ -52,7 +52,7 @@ message Book {
 
 - The resource **must** contain a `revision_id` field, which **should** be a
   string and contain a short, automatically-generated random string. A good
-  rule of thumb is the last eight characters of a UUID4.
+  rule of thumb is the last eight characters of a [UUID version 4 (random)](UUID4).
   - The `revision_id` field **must** document when new revisions are created
     (see [committing revisions](#committing-revisions) below).
   - The `revision_id` field **should** document the format of revision IDs.
@@ -412,3 +412,4 @@ used, and **must** clearly document it.
 [aip-142]: ./0142.md
 [aip-203]: ./0203.md
 [rfc 1738 §2.2]: https://tools.ietf.org/html/rfc1738
+[UUID4]: https://en.wikipedia.org/wiki/Universally_unique_identifier#Version_4_(random)