Commenting inside a tag is not possible.

But! You can define your own namespace in parent ViewGroup:

xmlns:comment="http://com.anything.comment"

and comment any property of View with:

<androidx.recyclerview.widget.RecyclerView
    .......
    .......
    
    comment_layoutManager="androidx.recyclerview.widget.LinearLayoutManager"
    comment_weight="1"
    comment_comment="Commented Text"
    tools_listitem="@layout/layout_voice_item"
    tools_itemCount="10"
/>

I can add a comment for a xml layout file using <!-- --> just like Code A.

Now I hope to add a comment in xml element <androidx.recyclerview.widget.RecyclerView>, but Code B doesn’t work, how can I fix it?

Code A

   <!-- This is a comment -->                          
    <androidx.recyclerview.widget.RecyclerView
        android_id="@+id/mvoice_list"
        android_layout_width="match_parent"
        android_layout_height="0dp"
        app_layout_constraintStart_toStartOf="parent"
        app_layout_constraintEnd_toEndOf="parent"
        app_layout_constraintBottom_toTopOf="@+id/button"
        app_layout_constraintTop_toBottomOf="@+id/adView"
        app_layoutManager="androidx.recyclerview.widget.LinearLayoutManager"
        tools_listitem="@layout/layout_voice_item"
        tools_itemCount="10"
    />

Code B

<androidx.recyclerview.widget.RecyclerView
    android_id="@+id/mvoice_list"
    android_layout_width="match_parent"
    android_layout_height="0dp"
    app_layout_constraintStart_toStartOf="parent"
    app_layout_constraintEnd_toEndOf="parent"
    app_layout_constraintBottom_toTopOf="@+id/button"
    app_layout_constraintTop_toBottomOf="@+id/adView"
    app_layoutManager="androidx.recyclerview.widget.LinearLayoutManager"
   
    <!-- This is a comment -->    
    tools_listitem="@layout/layout_voice_item"
    tools_itemCount="10"
/>