docs: Add a note about thin testers to the infra glossary

Clarifies the term "thin tester" which is kind of a sub category of
"child tester".

Bug: None
Change-Id: Ia83c8d90c6c1ea3d96fe8ea6b8969c0abd2c18dc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6203042
Reviewed-by: Struan Shrimpton <sshrimp@google.com>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1411787}

docs/infra/glossary.md

@@ -22,7 +22,8 @@ infrastructure.
     trigger builds on other builders. When the parent build only compiles and
     the child build only runs tests, we call this model a "builder/tester
     split". The child is referred to as the tester, and the parent (confusingly)
-    referred to as the builder.
+    referred to as the builder. The child might also be referred to as a "thin
+    tester" if it only runs tests remotely.
   * __CI Builder__ or __Post-submit Builder__: A builder that compiles/tests a
     branch of Chromium. Often triggered when changes to the branch are
     submitted, these post-submit builders run on a continuous basis.