docs: add some words about specifying conflicting extras/groups

This doesn't cover the optional `package` key since I wasn't quite sure
how to articulate its utility in a digestible way.

docs/concepts/dependencies.md

@@ -389,6 +389,12 @@ To add an optional dependency, use the `--optional <extra>` option:
 $ uv add httpx --optional network
 ```
 
+!!! note
+
+    If you have optional dependencies that conflict with one another and are using uv in
+    project mode with universal resolution, then resolution will fail. To work around this,
+    it's possible to [declare them as conflicting](./projects.md#optional-dependencies).
+
 ## Development dependencies
 
 Unlike optional dependencies, development dependencies are local-only and will _not_ be included in
@@ -455,8 +461,9 @@ to resolve the requirements of the project with an error.
 
 !!! note
 
-    There is currently no way to declare conflicting dependency groups. See
-    [astral.sh/uv#6981](https://github.com/astral-sh/uv/issues/6981) to track support.
+    If you have dependency groups that conflict with one another and are using uv in
+    project mode with universal resolution, then resolution will fail. To work around this,
+    it's possible to [declare them as conflicting](./projects.md#optional-dependencies).
 
 ### Default groups
 

docs/concepts/projects.md

@@ -484,10 +484,66 @@ each other and resolves all optional dependencies together when creating the loc
 If optional dependencies declared in one extra are not compatible with those in another extra, uv
 will fail to resolve the requirements of the project with an error.
 
-!!! note
+To work around this, uv supports declaring conflicting extras. For example, consider two sets of
+optional dependencies that conflict with one another:
+
+```toml title="pyproject.toml"
+[project.optional-dependencies]
+project1 = ["numpy==2.1.2"]
+project2 = ["numpy==2.0.0"]
+```
+
+If you try to run `uv lock` with the above dependencies, you should get an error like this:
+
+```
+  x No solution found when resolving dependencies:
+  `-> Because myproject[project2] depends on numpy==2.0.0 and myproject[project1] depends on numpy==2.1.2, we can conclude that myproject[project1] and
+      myproject[project2] are incompatible.
+      And because your project requires myproject[project1] and myproject[project2], we can conclude that your projects's requirements are unsatisfiable.
+```
+
+But if we specify that `project1` and `project2` are conflicting, uv will resolve them in different
+forks. We can specify conflicts in the `tool.uv` section:
+
+```toml title="pyproject.toml"
+[tool.uv]
+conflicts = [
+    [
+      { extra = "project1" },
+      { extra = "project2" },
+    ],
+]
+```
+
+And now running `uv lock` will succeed. Note though, that this will prevent you from enabling
+both `project1` and `project2` simultaneously:
+
+```console
+$ uv sync --extra project1 --extra project2
+Resolved 3 packages in 14ms
+error: extra `project1`, extra `project2` are incompatible with the declared conflicts: {`myproject[project1]`, `myproject[project2]`}
+```
+
+This error occurs because installing both `project1` and `project2` simultaneously could result
+in installing two different versions of the same package into the same environment.
+
+The above strategy for dealing with conflicting extras also works with groups:
+
+```toml title="pyproject.toml"
+[dependency-groups]
+project1 = ["numpy==2.1.2"]
+project2 = ["numpy==2.0.0"]
+
+[tool.uv]
+conflicts = [
+    [
+      { group = "project1" },
+      { group = "project2" },
+    ],
+]
+```
 
-    There is currently no way to declare conflicting optional dependencies. See
-    [astral.sh/uv#6981](https://github.com/astral-sh/uv/issues/6981) to track support.
+The only difference with conflicting extras is that you need to use `group` instead of `extra`.
 
 ## Managing dependencies