pw_sync: Add initial state of (Timed)ThreadNotification

Both ThreadNotification and TimedThreadNotification lack explicit
information about initial state. This CL adds this information to the
documentation.

Change-Id: I23421929e1b51ef5d1857add06ce9a5d186d6609
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/96882
Pigweed-Auto-Submit: Ewout van Bekkum <ewout@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>
Reviewed-by: Ewout van Bekkum <ewout@google.com>

pw_sync/docs.rst

@@ -1238,6 +1238,8 @@ backed by the most efficient native primitive for a target, regardless of
 whether that is a semaphore, event flag group, condition variable, or something
 else.
 
+The ThreadNotification is initialized to being empty (latch is not set).
+
 Generic BinarySemaphore-based Backend
 -------------------------------------
 This module provides a generic backend for ``pw::sync::ThreadNotification`` via
@@ -1362,6 +1364,8 @@ TimedThreadNotification
 The TimedThreadNotification is an extension of the ThreadNotification which
 offers timeout and deadline based semantics.
 
+The TimedThreadNotification is initialized to being empty (latch is not set).
+
 .. Warning::
   This is a single consumer/waiter, multiple producer/notifier API!
   The acquire APIs must only be invoked by a single consuming thread. As a

pw_sync/public/pw_sync/binary_semaphore.h

@@ -36,6 +36,8 @@ namespace pw::sync {
 // the user and/or backend MUST ensure that any initialization required in your
 // environment is done prior to the creation and/or initialization of the native
 // synchronization primitives (e.g. kernel initialization).
+//
+// The BinarySemaphore is initialized to being empty or having no tokens.
 class BinarySemaphore {
  public:
   using native_handle_type = backend::NativeBinarySemaphoreHandle;

pw_sync/public/pw_sync/counting_semaphore.h

@@ -37,6 +37,8 @@ namespace pw::sync {
 // the user and/or backend MUST ensure that any initialization required in your
 // environment is done prior to the creation and/or initialization of the native
 // synchronization primitives (e.g. kernel initialization).
+//
+// The CountingSemaphore is initialized to being empty or having no tokens.
 class CountingSemaphore {
  public:
   using native_handle_type = backend::NativeCountingSemaphoreHandle;

pw_sync/public/pw_sync/thread_notification.h

@@ -31,6 +31,8 @@ namespace pw::sync {
 //
 // The single consumer aspect of the API permits the use of a smaller and/or
 // faster native APIs such as direct thread signaling.
+//
+// The ThreadNotification is initialized to being empty (latch is not set).
 class ThreadNotification {
  public:
   using native_handle_type = backend::NativeThreadNotificationHandle;

pw_sync/public/pw_sync/timed_thread_notification.h

@@ -32,6 +32,8 @@ namespace pw::sync {
 //
 // The single consumer aspect of the API permits the use of a smaller and/or
 // faster native APIs such as direct thread signaling.
+//
+// The TimedThreadNotification is initialized to being empty (latch is not set).
 class TimedThreadNotification : public ThreadNotification {
  public:
   TimedThreadNotification() = default;