docs(env): CARGO_PRIMARY_PACKAGE always set for workspace members

Author: weihanglo

src/doc/src/reference/environment-variables.md

@@ -260,9 +260,10 @@ corresponding environment variable is set to the empty string, `""`.
 * `CARGO_PRIMARY_PACKAGE` --- This environment variable will be set if the
   package being built is primary. Primary packages are the ones the user
   selected on the command-line, either with `-p` flags or the defaults based
-  on the current directory and the default workspace members. This environment
-  variable will not be set when building dependencies. This is only set when
-  compiling the package (not when running binaries or tests).
+  on the current directory and the default workspace members.
+  This variable will not be set when building dependencies,
+  unless a dependency is also a workspace member.
+  This is only set when compiling the package (not when running binaries or tests).
 * `CARGO_TARGET_TMPDIR` --- Only set when building [integration test] or benchmark code.
   This is a path to a directory inside the target directory
   where integration tests or benchmarks are free to put any data needed by