Expand docs for Pubkey::create_program_address (#21750)

* Expand docs for Pubkey::create_program_address

* Update sdk/program/src/pubkey.rs

Co-authored-by: Tyera Eulberg <teulberg@gmail.com>

sdk/program/src/pubkey.rs

@@ -222,7 +222,11 @@ impl Pubkey {
     /// derived in off-chain client programs, avoiding the compute cost of
     /// generating the address on-chain. The address may or may not then be
     /// verified by re-deriving it on-chain, depending on the requirements of
-    /// the program.
+    /// the program. This verification may be performed without the overhead of
+    /// re-searching for the bump key by using the [`create_program_address`]
+    /// function.
+    ///
+    /// [`create_program_address`]: Pubkey::create_program_address
     ///
     /// **Warning**: Because of the way the seeds are hashed there is a potential
     /// for program address collisions for the same program id.  The seeds are
@@ -481,17 +485,48 @@ impl Pubkey {
         }
     }
 
-    /// Create a valid [program derived address][pda] without a bump seed.
+    /// Create a valid [program derived address][pda] without searching for a bump seed.
     ///
     /// [pda]: https://docs.solana.com/developing/programming-model/calling-between-programs#program-derived-addresses
     ///
-    /// **Because this function does not create a bump seed, it may unpredictably
-    /// return an error and should not be used. It exists for backwards
-    /// compatibility reasons.**
+    /// Because this function does not create a bump seed, it may unpredictably
+    /// return an error for any given set of seeds and is not generally suitable
+    /// for creating program derived addresses.
     ///
-    /// See the documentation for [`find_program_address`] for a full description.
+    /// However, it can be used for efficiently verifying that a set of seeds plus
+    /// bump seed generated by [`find_program_address`] derives a particular
+    /// address as expected. See the example for details.
+    ///
+    /// See the documentation for [`find_program_address`] for a full description
+    /// of program derived addresses and bump seeds.
     ///
     /// [`find_program_address`]: Pubkey::find_program_address
+    ///
+    /// # Examples
+    ///
+    /// Creating a program derived address involves iteratively searching for a
+    /// bump seed for which the derived [`Pubkey`] does not lie on the ed25519
+    /// curve. This search process is generally performed off-chain, with the
+    /// [`find_program_address`] function, after which the client passes the
+    /// bump seed to the program as instruction data.
+    ///
+    /// Depending on the application requirements, a program may wish to verify
+    /// that the set of seeds, plus the bump seed, do correctly generate an
+    /// expected address.
+    ///
+    /// The verification is performed by appending to the other seeds one
+    /// additional seed slice that contains the single `u8` bump seed, calling
+    /// `create_program_address`, checking that the return value is `Ok`, and
+    /// that the returned `Pubkey` has the expected value.
+    ///
+    /// ```
+    /// # use solana_program::pubkey::Pubkey;
+    /// # let program_id = Pubkey::new_unique();
+    /// let (expected_pda, bump_seed) = Pubkey::find_program_address(&[b"vault"], &program_id);
+    /// let actual_pda = Pubkey::create_program_address(&[b"vault", &[bump_seed]], &program_id)?;
+    /// assert_eq!(expected_pda, actual_pda);
+    /// # Ok::<(), anyhow::Error>(())
+    /// ```
     pub fn create_program_address(
         seeds: &[&[u8]],
         program_id: &Pubkey,