docs(ref): Clarify default-members and members relationship

The old statement that its a subset of `members` is incorrect because
some members can be inferred and not show up in `members`.

I tried to reword this to better convey the goal of what was being said

Author: epage

src/doc/src/reference/workspaces.md

@@ -127,7 +127,7 @@ a [virtual workspace](#virtual-workspace), it will apply to all members (as if
 
 ## The `default-members` field
 
-The `default-members` field specifies the members to
+The `default-members` field specifies paths of [members](#the-members-and-exclude-fields) to
 operate on when in the workspace root and the package selection flags are not
 used:
 
@@ -137,8 +137,6 @@ members = ["path/to/member1", "path/to/member2", "path/to/member3/*"]
 default-members = ["path/to/member2", "path/to/member3/foo"]
 ```
 
-When specified, `default-members` must expand to a subset of `members`.
-
 ## The `package` table
 
 The `workspace.package` table is where you define keys that can be