Add default member description in virtual workspace

Author: heisen-li

src/doc/src/reference/workspaces.md

@@ -39,16 +39,15 @@ To create a workspace, you add the `[workspace]` table to a `Cargo.toml`:
 # ...
 ```
 
-The directory where the workspace's Cargo.toml is located then becomes so-called the workspace root.
-
 At minimum, a workspace has to have a member, either with a root package or as
 a virtual manifest.
 
 ### Root package
 
 If the [`[workspace]` section](#the-workspace-section) is added to a
 `Cargo.toml` that already defines a `[package]`, the package is
-the *root package* of the workspace. 
+the *root package* of the workspace. The *workspace root* is the directory
+where the workspace's `Cargo.toml` is located.
 
 Example:
 ```toml
@@ -60,12 +59,13 @@ version = "0.1.0"    # the current version, obeying semver
 authors = ["Alice <[email protected]>", "Bob <[email protected]>"]
 ```
 
-Under [workspace](#the-workspace-section), you can specify which members the workspace has through 
-[workspace.members filed](#the-members-and-exclude-fields) .
+In the [worksapce section](#the-workspace-section), you can specify the members of 
+the workspace by [workspace.members filed](#the-members-and-exclude-fields). 
 
 If no other members are specified in [workspace.members filed](#the-members-and-exclude-fields) 
 in `Cargo.toml` in the workspace root directory, the root package will 
-become the default workspace member.
+become the default workspace member. In addition, you can also specify the default 
+workspace members through [workspace.default-members](#the-default-members-field).
 
 ### Virtual workspace
 
@@ -90,6 +90,10 @@ edition = "2021"     # the edition, will have no effect on a resolver used in th
 authors = ["Alice <[email protected]>", "Bob <[email protected]>"]
 ```
 
+In a virtual workspace, at least one member needed be included and if 
+[workspace.default-member](#the-default-members-field) is empty, all members of 
+the workspace are default members.
+
 Note that in a virtual manifest the [`resolver = "2"`](resolver.md#resolver-versions)
 should be specified manually. It is usually deduced from the [`package.edition`][package-edition]
 field which is absent in virtual manifests and the edition field of a member