remove mentions of `.bsn` asset format from docs for 0.19 or put warnings everywhere

[laundmo]

Current bsn docs mention the future .bsn asset format multiple times. Some of these docs have warnings, some don't.

Cases which i've found that don't have warnings:

bevy/crates/bevy_scene/src/spawn.rs

Lines 86 to 89 in a399db0

	/// // This scene inherits from the "player.bsn" asset. It will be spawned on the frame that "player.bsn"
	/// // is fully loaded.
	/// world.queue_spawn_scene(bsn! {
	/// :"player.bsn"

bevy/crates/bevy_scene/src/spawn.rs

Lines 167 to 171 in a399db0

	/// // This scene list inherits from the "player.bsn" asset. It will be spawned on the frame that "player.bsn"
	/// // is loaded.
	/// world.queue_spawn_scene_list(bsn_list! [
	/// (
	/// :"player.bsn"

bevy/crates/bevy_scene/src/spawn.rs

Lines 277 to 280 in a399db0

	/// // This scene inherits from the "player.bsn" asset. It will be spawned on the frame that "player.bsn"
	/// // is fully loaded.
	/// commands.queue_spawn_scene(bsn! {
	/// :"player.bsn"

bevy/crates/bevy_scene/src/spawn.rs

Line 319 in a399db0

/// :"player.bsn"

bevy/crates/bevy_scene/src/spawn.rs

Line 353 in a399db0

/// :"player.bsn"

[alice-i-cecile]

We have tests that stub this out and it's essential to understanding the right mental model. I would prefer warnings.