Make it more clear we have two types of workspaces

It needed a few rereads before I discovered the two flavours. Hope this helps.

src/doc/src/reference/workspaces.md

@@ -3,19 +3,26 @@
 A *workspace* is a collection of one or more packages that share common
 dependency resolution (with a shared `Cargo.lock`), output directory, and
 various settings such as profiles. Packages that are part of a workspaces are
-called *workspace members*.
+called *workspace members*. There are two flavours of workspaces: as root
+package or as virtual manifest.
+
+### Root package
 
 A workspace can be created by adding a [`[workspace]`
 section](#the-workspace-section) to `Cargo.toml`. This can be added to a
 `Cargo.toml` that already defines a `[package]`, in which case the package is
 the *root package* of the workspace. The *workspace root* is the directory
 where the workspace's `Cargo.toml` is located.
 
+### Virtual manifest
+
 Alternatively, a `Cargo.toml` file can be created with a `[workspace]` section
 but without a [`[package]` section][package]. This is called a *virtual
 manifest*. This is typically useful when there isn't a "primary" package, or
 you want to keep all the packages organized in separate directories.
 
+### Key features
+
 The key points of workspaces are:
 
 * All packages share a common `Cargo.lock` file which resides in the