add targeted help messages to `zombie_processes` diagnostic (#13760)

Fixes #13748

Diagnostic for that issue with this change:
```
warning: spawned process is not `wait()`ed on in all code paths
   --> x.rs:167:19
    |
167 |     let mut cmd = cmd.unwrap();
    |                   ^^^^^^^^^^^^
    |
note: no `wait()` call exists on the code path to this early return
   --> x.rs:178:47
    |
178 |             std::io::ErrorKind::BrokenPipe => return Some(0),
    |                                               ^^^^^^^^^^^^^^
note: `wait()` call exists, but it is unreachable due to the early return
   --> x.rs:185:10
    |
185 |     Some(cmd.wait().unwrap().code().unwrap()) // <-- wait()!
    |          ^^^
    = help: consider calling `.wait()` in all code paths
    = note: not doing so might leave behind zombie processes
    = note: see https://doc.rust-lang.org/stable/std/process/struct.Child.html#warning
```
Instead of saying "wait() is **never** called", it now says it's not
called by all code paths and points out the early return in particular.

changelog: none

0 files changed, 0 insertions, 0 deletions