gh-99238: clarify the type of the env dict. (#99253)

Author: gpshead

Doc/library/subprocess.rst

@@ -81,8 +81,10 @@ underlying :class:`Popen` interface can be used directly.
 
    If *env* is not ``None``, it must be a mapping that defines the environment
    variables for the new process; these are used instead of the default
-   behavior of inheriting the current process' environment. It is passed directly
-   to :class:`Popen`.
+   behavior of inheriting the current process' environment. It is passed
+   directly to :class:`Popen`. This mapping can be str to str on any platform
+   or bytes to bytes on POSIX platforms much like :data:`os.environ` or
+   :data:`os.environb`.
 
    Examples::
 
@@ -619,7 +621,9 @@ functions.
 
    If *env* is not ``None``, it must be a mapping that defines the environment
    variables for the new process; these are used instead of the default
-   behavior of inheriting the current process' environment.
+   behavior of inheriting the current process' environment. This mapping can be
+   str to str on any platform or bytes to bytes on POSIX platforms much like
+   :data:`os.environ` or :data:`os.environb`.
 
    .. note::