Shared overflow doc (Kotlin#2560)

* Clarify docs about the restriction on MutableSharedFlow function's onBufferOverflow parameter.

* Clarify docs about MutableSharedFlow function without subscribers but with onBufferOverflow

Co-authored-by: Zach Klippenstein <zachklipp@squareup.com>

kotlinx-coroutines-core/common/src/flow/SharedFlow.kt

@@ -68,6 +68,13 @@ import kotlin.native.concurrent.*
  * the `onBufferOverflow` parameter, which is equal to one of the entries of the [BufferOverflow] enum. When a strategy other
  * than [SUSPENDED][BufferOverflow.SUSPEND] is configured, emissions to the shared flow never suspend.
  *
+ * **Buffer overflow condition can happen only when there is at least one subscriber that is not ready to accept
+ * the new value.**  In the absence of subscribers only the most recent `replay` values are stored and the buffer
+ * overflow behavior is never triggered and has no effect. In particular, in the absence of subscribers emitter never
+ * suspends despite [BufferOverflow.SUSPEND] option and [BufferOverflow.DROP_LATEST] option does not have effect either.
+ * Essentially, the behavior in the absence of subscribers is always similar to [BufferOverflow.DROP_OLDEST],
+ * but the buffer is just of `replay` size (without any `extraBufferCapacity`).
+ *
  * ### Unbuffered shared flow
  *
  * A default implementation of a shared flow that is created with `MutableSharedFlow()` constructor function
@@ -221,9 +228,12 @@ public interface MutableSharedFlow<T> : SharedFlow<T>, FlowCollector<T> {
  * @param replay the number of values replayed to new subscribers (cannot be negative, defaults to zero).
  * @param extraBufferCapacity the number of values buffered in addition to `replay`.
  *   [emit][MutableSharedFlow.emit] does not suspend while there is a buffer space remaining (optional, cannot be negative, defaults to zero).
- * @param onBufferOverflow configures an action on buffer overflow (optional, defaults to
- *   [suspending][BufferOverflow.SUSPEND] attempts to [emit][MutableSharedFlow.emit] a value,
- *   supported only when `replay > 0` or `extraBufferCapacity > 0`).
+ * @param onBufferOverflow configures an [emit][MutableSharedFlow.emit] action on buffer overflow. Optional, defaults to
+ *   [suspending][BufferOverflow.SUSPEND] attempts to emit a value.
+ *   Values other than [BufferOverflow.SUSPEND] are supported only when `replay > 0` or `extraBufferCapacity > 0`.
+ *   **Buffer overflow can happen only when there is at least one subscriber that is not ready to accept
+ *   the new value.** In the absence of subscribers only the most recent [replay] values are stored and
+ *   the buffer overflow behavior is never triggered and has no effect.
  */
 @Suppress("FunctionName", "UNCHECKED_CAST")
 public fun <T> MutableSharedFlow(