[descriptor] Make the syntax of descriptor!() more consistent

The syntax now is pretty much the same as the normal descriptor syntax,
with the only difference that modifiers cannot be grouped together (i.e.
`sdv:older(144)` must be turned into `s:d:v:older(144)`.

src/descriptor/mod.rs

@@ -42,7 +42,8 @@ pub use miniscript::{
 };
 
 pub mod checksum;
-mod dsl;
+#[doc(hidden)]
+pub mod dsl;
 pub mod error;
 pub mod policy;
 pub mod template;