docs: add documents for #[rustsbi(dynamic)].

Signed-off-by: Zhenchen Wang <m202372036@hust.edu.cn>

src/lib.rs

@@ -866,6 +866,44 @@ pub use sbi_spec::binary::{HartMask, Physical, SbiRet, SharedPtr};
 /// # }
 /// ```
 ///
+/// When using the `#[rustsbi(dynamic)]` attribute, it is possible to include multiple
+/// fields that indicate the same SBI extension.
+///
+/// ```rust
+/// # use rustsbi::RustSBI;
+/// #[derive(RustSBI)]
+/// #[rustsbi(dynamic)]
+/// struct MySBI {
+///     // Fields in `#[rustsbi(dynamic)]` structures are usually wrapped in `Option`s.
+///     fence: Option<MyFence>,
+///     rfnc: Option<MyFence>,
+///     // ^ Both the two fields are identified as `rustsbi::Fence` implementation.
+///     info: MyEnvInfo,
+///     // ^ However, environment information should only be introduced *once*.
+/// }
+///
+/// // RustSBI will sequentially examine these fields, starting from the first one.
+/// // Upon encountering the first `Option` that is `Some`, it will utilize this
+/// // field as the implementation.
+/// // i.e., if the first field `fence` in `MySBI` is `Some`, RustSBI uses `fence`.
+/// // Else, if the second field `rfnc` is `Some`, RustSBI uses `rfnc`.
+/// // Otherwise, RustSBI returns `SbiRet::not_supported`.
+///
+/// # use sbi_spec::binary::{SbiRet, HartMask};
+/// # struct MyFence;
+/// # impl rustsbi::Fence for MyFence {
+/// #     fn remote_fence_i(&self, _: HartMask) -> SbiRet { unimplemented!() }
+/// #     fn remote_sfence_vma(&self, _: HartMask, _: usize, _: usize) -> SbiRet { unimplemented!() }
+/// #     fn remote_sfence_vma_asid(&self, _: HartMask, _: usize, _: usize, _: usize) -> SbiRet { unimplemented!() }
+/// # }
+/// # struct MyEnvInfo;
+/// # impl rustsbi::EnvInfo for MyEnvInfo {
+/// #     fn mvendorid(&self) -> usize { 1 }
+/// #     fn marchid(&self) -> usize { 2 }
+/// #     fn mimpid(&self) -> usize { 3 }
+/// # }
+/// ```
+///
 /// The struct as derive input may include generics, specifically type generics, lifetimes,
 /// constant generics and where clauses.
 ///