Object3D: Add note about traversal in docs

[mjurczyk]

Based on:
"After deleting the child object, the code cannot be executed" (Removed descendant during traverse)
"Gltf.scene.traverse get TypeError: Cannot read property ‘traverse’ of undefined" (Moved descendant to another parent during traverse)
"Issue with adding and remove items from scene" (Moved descendant to another parent during traverse)
"Add children to a group break traverse method" (Moved descendant to another parent during traverse)
"Super weird behavior with “add()” and looping “scene.children”" (Moved descendant to another parent during traverse)

Adding clarification in the docs could save some confusion, since it is not clear without looking at the code that there's no safe-check on descendant#traverse on this line (edit: we could also just add a safe-check, while we're at it? 🤔 )

[mrdoob]

Better to use <strong> instead of <b>.

[mrdoob]

Thanks!