[Bugfix] Fix failure to launch in Tensor Parallel TP mode on macOS. (#14948)

Signed-off-by: Kebe <mail@kebe7jun.com>
Signed-off-by: youkaichao <youkaichao@gmail.com>
Co-authored-by: youkaichao <youkaichao@gmail.com>

docs/source/design/multiprocessing.md

@@ -24,7 +24,7 @@ This document describes how vLLM deals with these challenges.
 [Python multiprocessing methods](https://docs.python.org/3/library/multiprocessing.html#contexts-and-start-methods) include:
 
 - `spawn` - spawn a new Python process. This will be the default as of Python
-  3.14.
+  3.14. In macOS, this is already the default.
 
 - `fork` - Use `os.fork()` to fork the Python interpreter. This is the default
   in Python versions prior to 3.14.
@@ -34,7 +34,7 @@ This document describes how vLLM deals with these challenges.
 ### Tradeoffs
 
 `fork` is the fastest method, but is incompatible with dependencies that use
-threads.
+threads. If you are under macOS, using `fork` may cause the process to crash.
 
 `spawn` is more compatible with dependencies, but can be problematic when vLLM
 is used as a library. If the consuming code does not use a `__main__` guard (`if