Make `LegacyScriptPubKeyMan::CanProvide` method able to recognize p2sh scripts when the redeem script is present in the `mapScripts` map without the p2sh script also having to be added to the `mapScripts` map. This restores behavior prior to #17261, which I think broke backwards compatibility with old wallet files by no longer treating addresses created by `addmultisigaddress` calls before #17261 as solvable.

The reason why tests didn't fail with the CanProvide implementation in #17261 is because of a workaround added in 4a7e43e8460127a40a7895519587399feff3b682 "Store p2sh scripts in AddAndGetDestinationForScript", which masked the problem for new `addmultisigaddress` RPC calls without fixing it for multisig addresses already created in old wallet files.

This change adds a lot of comments and allows reverting commit 4a7e43e8460127a40a7895519587399feff3b682 "Store p2sh scripts in AddAndGetDestinationForScript", so the `AddAndGetDestinationForScript()` function, `CanProvide()` method, and `mapScripts` map should all be more comprehensible

ninja all check check-functional