documentation: clarify Broadcasting section

Each prior section references what RL struct satisfies the given requirement for a functional lightning node, so seems it would make sense for this section to reference something about the BroadcasterInterface?

Read the documentation's Broadcasting section and compare its structure with the preceding sections that identify the relevant RL struct. Check how BroadcasterInterface is described, then clarify the section so it explicitly references the appropriate requirement or implementation; done means the section is consistent and understandable alongside the others.

Written by the indexing model from the issue text.