docs: add backward incompat note about #10875

Fixes #11002

CHANGELOG.md

@@ -1,5 +1,9 @@
 ## 1.1.3 (July 29, 2021)
 
+__BACKWARDS INCOMPATIBILITIES:__
+
+* api: The Job Run and Plan APIs now use the `?namespace=` query parameter before the namespace from the job. This matches region's behavior. Users of `api.Client` should ensure their `Config.Namespace` is unset if they want to use the namespace in the job. [[GH-10875](https://github.com/hashicorp/nomad/issues/10875)]
+
 IMPROVEMENTS:
 
 * api: Added `NewSystemJob` helper function to create base system job object. [[GH-10861](https://github.com/hashicorp/nomad/issues/10861)]
@@ -172,6 +176,10 @@ BUG FIXES:
 
 ## 1.0.9 (July 29, 2021)
 
+__BACKWARDS INCOMPATIBILITIES:__
+
+* api: The Job Run and Plan APIs now use the `?namespace=` query parameter before the namespace from the job. This matches region's behavior. Users of `api.Client` should ensure their `Config.Namespace` is unset if they want to use the namespace in the job. [[GH-10875](https://github.com/hashicorp/nomad/issues/10875)]
+
 BUG FIXES:
 
 * core: Fixed a bug where internalized constraint strings broke job plan [[GH-10896](https://github.com/hashicorp/nomad/issues/10896)]

website/content/docs/upgrade/upgrade-specific.mdx

@@ -13,13 +13,25 @@ upgrade. However, specific versions of Nomad may have more details provided for
 their upgrades as a result of new features or changed behavior. This page is
 used to document those details separately from the standard upgrade flow.
 
-## Nomad 1.1.3
+## Nomad 1.0.9 and 1.1.3
+
+#### Namespace in Job Run and Plan APIs
+
+The Job Run and Plan APIs now respect the `?namespace=...` query parameter over
+the namespace specified in the job itself. This matches the precedence of
+region and [fixes a bug where the `-namespace` flag was not respected for the
+`nomad run` and `nomad apply` commands.][gh-10875]
+
+For users of [`api.Client`][go-client] who want their job namespace respected,
+you must ensure the `Config.Namespace` field is unset.
 
 #### Docker Driver
 
-Starting in Nomad 1.1.2, task groups with `network.mode = "bridge"` generated
-a hosts file in Docker containers. This generated hosts file was bind-mounted
-from the task directory to `/etc/hosts` within the task. In Nomad 1.1.3 the
+**1.1.3 only**
+
+Starting in Nomad 1.1.2, task groups with `network.mode = "bridge"` generated a
+hosts file in Docker containers. This generated hosts file was bind-mounted
+from the task directory to `/etc/hosts` within the task.  In Nomad 1.1.3 the
 source for the bind mount was moved to the allocation directory so that it is
 shared between all tasks in an allocation.
 
@@ -1128,6 +1140,8 @@ deleted and then Nomad 0.3.0 can be launched.
 [gh-6787]: https://github.com/hashicorp/nomad/issues/6787
 [gh-8457]: https://github.com/hashicorp/nomad/issues/8457
 [gh-9148]: https://github.com/hashicorp/nomad/issues/9148
+[gh-10875]: https://github.com/hashicorp/nomad/pull/10875
+[go-client]: https://pkg.go.dev/github.com/hashicorp/nomad/api#Client
 [hcl2]: https://github.com/hashicorp/hcl2
 [limits]: /docs/configuration#limits
 [lxc]: /docs/drivers/external/lxc