doc: update core_thread_start() documentation

It is now passed an arugment to pass to the callback
Reviewed-by: NMatt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15278)

doc/man7/provider-base.pod

@@ -21,7 +21,8 @@ provider-base
 
  typedef void (*OSSL_thread_stop_handler_fn)(void *arg);
  int core_thread_start(const OSSL_CORE_HANDLE *handle,
-                       OSSL_thread_stop_handler_fn handfn);
+                       OSSL_thread_stop_handler_fn handfn,
+                       void *arg);
 
  OPENSSL_CORE_CTX *core_get_libctx(const OSSL_CORE_HANDLE *handle);
  void core_new_error(const OSSL_CORE_HANDLE *handle);
@@ -192,13 +193,14 @@ core_get_params() retrieves parameters from the core for the given I<handle>.
 See L</Core parameters> below for a description of currently known
 parameters.
 
-The core_thread_start() function informs the core that the provider has started
+The core_thread_start() function informs the core that the provider has stated
 an interest in the current thread. The core will inform the provider when the
 thread eventually stops. It must be passed the I<handle> for this provider, as
 well as a callback I<handfn> which will be called when the thread stops. The
-callback will subsequently be called from the thread that is stopping and gets
-passed the provider context as an argument. This may be useful to perform thread
-specific clean up such as freeing thread local variables.
+callback will subsequently be called, with the supplied argument I<arg>, from
+the thread that is stopping and gets passed the provider context as an
+argument. This may be useful to perform thread specific clean up such as
+freeing thread local variables.
 
 core_get_libctx() retrieves the library context in which the library
 object for the current provider is stored, accessible through the I<handle>.