SDK doc fixes (#3133)

* fix decode-error doc links

* fix redundant link in program-option

* mark as source code to avoid unclosed HTML tag warning

* fix url

sdk/decode-error/src/lib.rs

@@ -18,8 +18,8 @@ use num_traits::FromPrimitive;
 /// `FromPrimitive`, it is only used correctly when the caller is certain of the
 /// original error type.
 ///
-/// [`ProgramError`]: crate::program_error::ProgramError
-/// [`ProgramError::Custom`]: crate::program_error::ProgramError::Custom
+/// [`ProgramError`]: https://docs.rs/solana-program-error/latest/solana_program_error/enum.ProgramError.html
+/// [`ProgramError::Custom`]: https://docs.rs/solana-program-error/latest/solana_program_error/enum.ProgramError.html#variant.Custom
 /// [`ToPrimitive`]: num_traits::ToPrimitive
 pub trait DecodeError<E> {
     fn decode_custom_error_to_enum(custom: u32) -> Option<E>

sdk/program-option/src/lib.rs

@@ -882,7 +882,7 @@ impl<T: Clone> Clone for COption<T> {
 }
 
 impl<T> Default for COption<T> {
-    /// Returns [`COption::None`][COption::None].
+    /// Returns [`COption::None`]
     ///
     /// # Examples
     ///

sdk/src/signer/signers.rs

@@ -17,8 +17,8 @@ pub trait Signers {
 /// Any `T` where `T` impls `IntoIterator` yielding
 /// `Signer`s implements `Signers`.
 ///
-/// This includes [&dyn Signer], [Box<dyn Signer>],
-/// [&dyn Signer; N], Vec<dyn Signer>, Vec<Keypair>, etc.
+/// This includes `[&dyn Signer]`, `[Box<dyn Signer>]`,
+/// `[&dyn Signer; N]`, `Vec<dyn Signer>`, `Vec<Keypair>`, etc.
 ///
 /// When used as a generic function param, `&T`
 /// should be used instead of `T` where T: Signers, due to the `?Sized` bounds on T.