[swarming] Clarify bot configuration for chromium-style docker bots.

R=bpastene@chromium.org, steveroe@google.com, vadimsh@chromium.org

Change-Id: I64868a84a9b364a497f6cb76f72e895559b46a02
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/1918610
Reviewed-by: Vadim Shtayura <vadimsh@chromium.org>
Commit-Queue: Vadim Shtayura <vadimsh@chromium.org>
Auto-Submit: Robbie Iannucci <iannucci@chromium.org>
diff --git a/appengine/swarming/proto/config/bots.proto b/appengine/swarming/proto/config/bots.proto
index 08e1ca3..86cd61f 100644
--- a/appengine/swarming/proto/config/bots.proto
+++ b/appengine/swarming/proto/config/bots.proto
@@ -65,6 +65,21 @@
 // If bot_id and bot_id_prefix are both missing, the group defines all bots that
 // didn't fit into other groups. There can be only one such "default" group.
+// NOTE: Swarming allows 'derivative' bots where multiple bots run on the same
+// host machine in some sort of container (such as Docker containers) and SHARE
+// A SINGLE CREDENTIAL. These bots have hostnames such as 'hostmachine--001',
+// where the "--001" suffix indicates which of the contained bots on
+// 'hostmachine' it is. When Swarming checks the bot affiliation for this
+// contained machine, it checks ONLY the 'hostmachine' portion. This means that
+// if your contained bot is 'vm10-vlan2--001', it would match:
+//   `bot_id: "vm10-vlan2`
+//   `bot_id: "vm{0...100}-vlan2`
+//   `bot_id_prefix: "vm10-vlan`
+// but would NOT match:
+//   `bot_id: "vm10-vlan2--001`
+//   `bot_id_prefix: "vm10-vlan2-`
+//   `bot_id_prefix: "vm10-vlan2--`
 // TODO(vadimsh): Introduce explicit field "use_as_default" instead.
 message BotGroup {
   reserved 3;