Clarify thread-safety of SharedFlow methods in docs

[elizarov]

• Override MutableSharedFlow.emit to attach a more appropriate docs than the one inherited from FlowCollector.
• Clarify thread-safety of all the MutableSharedFlow & MutableState "mutating" methods.

The latter is needed, because Flows, in general, are sequential, but shared flows provide all the necessarily synchronization themselves, so, to avoid confusion it makes sense to additionally mention thread-safety of shared flows in all the relevant mutating functions.

[qwwdfsad]

A bit weird, because we already have an explicit mention of this behaviour in class documentation

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

Line 100 in d4c3c9c

* All methods of shared flow are **thread-safe** and can be safely invoked from concurrent coroutines without

E.g. we do not expect to have each method of ConcurrentHashMap to be documented as thread-safe, do we?

I don't really mind, it just a bit excessive

[Tolriq]

@qwwdfsad HashMap method does not explicitly have methods with docs that says it's not thread safe. Here IDE on emit says in big not thread safe making people doubt.

[elizarov]

I believe that excessiveness is warranted here, because a regular Flow is sequential and, in general, a FlowCollector.emit cannot be used concurrently. SharedFlow is an exception and stressing this difference is important (not everyone will read long SharedFlow doc in full).