Separate use cases from non-use cases of workspaces

Partially address astral-sh#5605 (comment)

docs/concepts/workspaces.md

@@ -147,7 +147,7 @@ albatross
 Since `seeds` was excluded in the `pyproject.toml`, the workspace has two members total: `albatross`
 (the root) and `bird-feeder`.
 
-## When (not) to use workspaces
+## Summary of use cases
 
 Workspaces are intended to facilitate the development of multiple interconnected packages within a
 single repository. As a codebase grows in complexity, it can be helpful to split it into smaller,
@@ -164,6 +164,8 @@ Other common use cases for workspaces include:
 - A library with a plugin system, where each plugin is a separate workspace package with a
   dependency on the root.
 
+## When _not_ to use workspaces
+
 Workspaces are _not_ suited for cases in which members have conflicting requirements, or desire a
 separate virtual environment for each member. In this case, path dependencies are often preferable.
 For example, rather than grouping `albatross` and its members in a workspace, you can always define