f0cec015b59ab484e81ca675d1170af5eb553911 Add small clarification to docs (thunderbiscuit)
Pull request description:
### Description
Very small fixes to documentation:
1. I got a DM last week from a user who thought we had a bug with our timestamps. It turns out he was using the milliseconds version of the Unix timestamp in his project and didn't realize we were giving out a standard Unix timestamp. The docs now mention this explicitly.
2. I noticed some small inconsistencies in the documentation on the public templates while porting them over to Kotlin. This PR also fixes that so that all templates use a common documentation wording.
### Checklists
#### All Submissions:
* [x] I've signed all my commits
* [x] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk/blob/master/CONTRIBUTING.md)
* [x] I ran `cargo fmt` and `cargo clippy` before committing
ACKs for top commit:
notmandatory:
Re-ACK f0cec015b59ab484e81ca675d1170af5eb553911
Tree-SHA512: ad42278126a0613fb1ba15f4e0ca92e05038389ac2e6b1015ea045f30ee8e92a40d6c089c35d0492bba0dc6d71e44b29879bd37a8bc491ff6367a89cab958db2
Clippy emits:
warning: Question mark operator is useless here
No need to use the `?` operator inside an `Ok()` statement when
returning, just return directly.
A new `ExtendedKey` type has been added, which is basically an enum of
`bip32::ExtendedPubKey` and `bip32::ExtendedPrivKey`, with some extra metadata
regarding the `ScriptContext`.
This type has some methods that make it very easy to extract its content as
either an `xprv` or `xpub`.
The `DerivableKey` trait has been updated so that the user now only has to
implement a method (`DerivableKey::into_extended_key()`) to perform the
conversion into an `ExtendedKey`.
The method that was previously called `add_metadata()` has now been renamed
to `into_descriptor_key()`, and it has
a blanket implementation.
Change the return type of the `descriptor!()` macro and `ToWalletDescriptor` to
avoid having to map errors.
Also introduce more checks to validate descriptors built using the macro.
Thanks to the `ToWalletDescriptor` trait we can also very easily validate the checksum
for descriptors that are loaded from strings, if they contain one. Fixes#20.
