Doc improvements

[notmandatory]

bdk_chain

• CheckPoint's doc comment could mention it's guaranteed to always have at least one element, since it's an invariant relied upon across the project.
• merge_chains's doc comment is outdated.
• The doc comment for the rusqlite_impl module is a placeholder: //! Module for stuff.
• Like walk_ancestors, walk_descendants's doc could mention it excludes the root tx from the iterator.
• In SpkIterator::next(), derived_descriptor's descriptor is unwrapped because "the descriptor cannot need hardened derivation". It'd be better to have "we check it's never hardened in the constructor" as it's a stronger invariant. (Mentioning as I initially missed it was also checked in SpkIterator::new_with_range() by re-assigning to end the minimum between end and BIP32_MAX_INDEX.)

bdk_esplora

• Top-level documentation states "A stop_gap of 0 will be treated as a stop_gap of 1" but it's not the case. After making parallel_requests requests in the first iteration of the loop, even if all indexes were detected as active, gap_limit_reached would be set to true if stop_gap is set to 0. Not only would it not be the case if stop_gap was set to 1, but it's also surprising to stop whereas there was no gap.

[oleonardolima]

I'd suggest changing this issue to have checkmarks, so we can check the ones already fixed, such as:

• The doc comment for the rusqlite_impl module is a placeholder: //! Module for stuff. (fixed by Sqlite - allow persisting anchor without tx bdk#1736)