Clarifies the dev-dependencies -> group.dev.dependencies switch

As written, it seemed to prioritize the older section notation without clarifying that the older syntax still works but shouldn't be used. This reorder visually prioritizes the preferred syntax as of 1.2.x while clearly explaining what the old syntax is during this transition period.

The best next step here would be a clear definition of the version in which dev-dependencies will go away.

docs/managing-dependencies.md

@@ -59,19 +59,24 @@ use [extras]({{< relref "pyproject#extras" >}}) instead. Extras can be installed
 {{% /note %}}
 
 {{% note %}}
-**A note about the `dev-dependencies` section**
+**A note about defining a `dev` dependencies group**
 
-Any dependency declared in the `dev-dependencies` section will automatically be added to a `dev` group.
-So the two following notations are equivalent:
+The proper way to define a `dev` dependencies group as of Poetry 1.2.x is the following:
 
 ```toml
-[tool.poetry.dev-dependencies]
+[tool.poetry.group.dev.dependencies]
 pytest = "^6.0.0"
 pytest-mock = "*"
 ```
 
+This group notation is preferred for Poetry 1.2.x and not usable in earlier versions.
+For backwards compatibility with older versions of Poetry,
+any dependency declared in the `dev-dependencies` section will automatically be added to the `dev` group.
+So the above and following notations are equivalent:
+
 ```toml
-[tool.poetry.group.dev.dependencies]
+# Poetry pre-1.2.x style, understood by Poetry 1.0–1.2
+[tool.poetry.dev-dependencies]
 pytest = "^6.0.0"
 pytest-mock = "*"
 ```