Fix TODO comments throughout the solution

[LSViana]

This list is not final and will be accumulated as items are solved:

• Make it easier to connect OutputNative size with pointer size in MapEntryNative
• Remove mentions about *Native counterpart for event classes that don't need it
• Fix tests for XmlElement.Observe() that weren't using a root-level XmlElement but should be
• Fix tests for XmlElement.Unobserve() that weren't using a root-level XmlElement but should be
• Fix tests for XmlElement.Parent() that weren't returning the correct value
• Fix tests for XmlElement.String() that weren't testing for child XmlText nodes but should be
• Remove mentions about read-only transactions on writeable functions (they will be tested later, after exception handling is added)
• Remove mention to test Transaction.Dispose()
• Remove comment from Output.Object
• Remove comment from Map.Length()
• Remove comment from TextChannel
• Remove comment for Doc.Guid that will not be fixed because the GUID format isn't compatible with C#
• Remove comment from InputNative because the size of the data can't be reduced or data will be lost
• Fix the constructor of Output to support null pointer
  • Update usages of new Output() to use ReferenceAccessor.Access(Output)
• Implement test of getting null value from Map after fixing the Output constructor
• Replace NotImplemented with NotSupported exception on the EventBranch constructor since all cases are now covered
• Implement remaining tests marked as [Ignore] because they were waiting for implementations
• Implement IEnumerator<T>.Reset() on the following classes:
  • ArrayEnumerator
  • MapEnumerator
  • XmlAttributeEnumerator
  • XmlTreeWalkerEnumerator
  • Remove the documentation mention about these classes only supporting one-time enumeration
• Wrap the XmlElement with an XmlFragment before returning the value.
• Wrap the XmlText with an XmlFragment before returning the value.
• Dispose EventSubscription with Unsubscribe().

[SebastianStehle]

@LSViana We have this TODO in the code:

    // TODO This is a temporary solution to track the amount of transactions a document has open.
    // It's fragile because a cloned instance of the same document won't be synchronized and if a `Transaction`
    // is opened in the cloned instance, it'll receive `null` from the Rust side and will cause the
    // `ThrowHelper.PendingTransaction()` to run (which is acceptable since it's a managed exception).
    internal void NotifyTransactionStarted()
    {
        openTransactions++;
    }

    internal void NotifyTransactionClosed()
    {
        openTransactions--;
    }
    ```
    
    
    Do you think we will ever have a final solution or shall we just remove this and also close this issue?

[LSViana]

@SebastianStehle I think we'll have some solutions if we ever look into that in detail but I didn't do it yet.

Given that, I'd just leave it there. It's not harmful and informs other developers who read the same code which is a positive detail.