docs: better document `default` and `wasm` features on `leptos_axum` (closes #1872) #1006

Job	Run time
get-leptos-changed / Detect Source Change	7s
CI (integrations/actix) / Run ci (nightly)	14m 6s
CI (integrations/axum) / Run ci (nightly)	14m 3s
CI (integrations/utils) / Run ci (nightly)	14m 5s
CI (integrations/viz) / Run ci (nightly)	14m 4s
CI (leptos) / Run ci (nightly)	14m 3s
CI (leptos_config) / Run ci (nightly)	14m 4s
CI (leptos_dom) / Run ci (nightly)	14m 3s
CI (leptos_hot_reload) / Run ci (nightly)	14m 1s
CI (leptos_macro) / Run ci (nightly)	14m 5s
CI (leptos_reactive) / Run ci (nightly)	14m 4s
CI (leptos_server) / Run ci (nightly)	14m 3s
CI (meta) / Run ci (nightly)	14m 3s
CI (router) / Run ci (nightly)	14m 1s
CI (server_fn) / Run ci (nightly)	14m 2s
CI (server_fn_macro) / Run ci (nightly)	14m 1s
CI (server_fn/server_fn_macro_default) / Run ci (nightly)	13m 42s
	3h 44m 37s

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