Change `#[cfg(docsrs)]` to `#[cfg(doc)]` on `use`

The additional `use` items were added to improve the style of
documentation. Because they were only used for doc they were `cfg`ed.
But because this is independent from being built by `docs.rs` the `cfg`
should've been `doc` not `docsrs`.

IOW `docsrs` means roughly `all(doc, nightly)` and the added items are
unrelated to `nightly`.

bitcoin/src/blockdata/locktime/absolute.rs

@@ -24,7 +24,7 @@ use crate::prelude::*;
 use crate::parse::{self, impl_parse_str_through_int};
 use crate::string::FromHexStr;
 
-#[cfg(docsrs)]
+#[cfg(doc)]
 use crate::absolute;
 
 /// The Threshold for deciding whether a lock time value is a height or a time (see [Bitcoin Core]).

bitcoin/src/blockdata/locktime/relative.rs

@@ -13,7 +13,7 @@ use core::convert::TryFrom;
 #[cfg(all(test, mutate))]
 use mutagen::mutate;
 
-#[cfg(docsrs)]
+#[cfg(doc)]
 use crate::relative;
 
 /// A relative lock time value, representing either a block height or time (512 second intervals).