Auto merge of #12388 - zvolin:docs-workspace-resolver, r=epage

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 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.

This can raise the awareness of this behavior and prevent issues like #12387. I also wasn't aware of this behavior before while not being so new to Rust, and we have the resolver 2 for good reasons, so I think we should be more explicit about it in the documentation.
When someone looks for the 'how to make cargo workspace' answers, he's unlikely to get to the `Dependency Resolution` section at the same time, he'll likely just copy paste the workspace example from the `Workspaces` and call it a day, yet extending the usage of an old resolver and not benefiting from the new one.

Author: bors

src/doc/src/reference/workspaces.md

@@ -67,16 +67,23 @@ you want to keep all the packages organized in separate directories.
 # [PROJECT_DIR]/Cargo.toml
 [workspace]
 members = ["hello_world"]
+resolver = "2"
 ```
 
 ```toml
 # [PROJECT_DIR]/hello_world/Cargo.toml
 [package]
 name = "hello_world" # the name of the package
 version = "0.1.0"    # the current version, obeying semver
+edition = "2021"     # the edition, will have no effect on a resolver used in the workspace
 authors = ["Alice <[email protected]>", "Bob <[email protected]>"]
 ```
 
+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
+won't affect the resolver used by the workspace.
+
 ### The `members` and `exclude` fields 
 
 The `members` and `exclude` fields define which packages are members of
@@ -241,6 +248,7 @@ if that makes sense for the tool in question.
 [package]: manifest.md#the-package-section
 [`Cargo.lock`]: ../guide/cargo-toml-vs-cargo-lock.md
 [package-metadata]: manifest.md#the-metadata-table
+[package-edition]: manifest.md#the-edition-field
 [output directory]: ../guide/build-cache.md
 [patch]: overriding-dependencies.md#the-patch-section
 [replace]: overriding-dependencies.md#the-replace-section