docs: raise awareness of resolver used inside workspace

Workspaces by default use the resolver in version 1. It's been
some time already since the 2021 edition which made version 2
a default for the alone packages, but yet most of the projects
that make a use of the workspaces still depends on an old resolver.

By being explicit about the resolver tag inside the workspace
we can lower the usage of older resolver for the new projects.

Author: zvolin

src/doc/src/reference/workspaces.md

@@ -39,6 +39,9 @@ To create a workspace, you add the `[workspace]` table to a `Cargo.toml`:
 
 At minimum, a workspace has to have a member, either with a root package or as
 a virtual manifest.
+It's also a good practice to specify the `resolver = "2"` unless it is necessary
+to rely on the old one. It is a default resolver for all packages in the `2021` edition
+but it has to be explicitely set under the workspace.
 
 #### Root package
 
@@ -49,6 +52,7 @@ where the workspace's `Cargo.toml` is located.
 
 ```toml
 [workspace]
+resolver = "2"
 
 [package]
 name = "hello_world" # the name of the package
@@ -67,6 +71,7 @@ you want to keep all the packages organized in separate directories.
 # [PROJECT_DIR]/Cargo.toml
 [workspace]
 members = ["hello_world"]
+resolver = "2"
 ```
 
 ```toml
@@ -86,6 +91,7 @@ the workspace:
 [workspace]
 members = ["member1", "path/to/member2", "crates/*"]
 exclude = ["crates/foo", "path/to/other"]
+resolver = "2"
 ```
 
 All [`path` dependencies] residing in the workspace directory automatically
@@ -127,6 +133,7 @@ used:
 [workspace]
 members = ["path/to/member1", "path/to/member2", "path/to/member3/*"]
 default-members = ["path/to/member2", "path/to/member3/foo"]
+resolver = "2"
 ```
 
 When specified, `default-members` must expand to a subset of `members`.
@@ -158,6 +165,7 @@ Example:
 # [PROJECT_DIR]/Cargo.toml
 [workspace]
 members = ["bar"]
+resolver = "2"
 
 [workspace.package]
 version = "1.2.3"
@@ -192,6 +200,7 @@ Example:
 # [PROJECT_DIR]/Cargo.toml
 [workspace]
 members = ["bar"]
+resolver = "2"
 
 [workspace.dependencies]
 cc = "1.0.73"
@@ -224,6 +233,7 @@ configuration in `Cargo.toml`. For example:
 ```toml
 [workspace]
 members = ["member1", "member2"]
+resolver = "2"
 
 [workspace.metadata.webcontents]
 root = "path/to/webproject"