Update runtime.md (#7568)

book/src/validator/runtime.md

@@ -38,6 +38,10 @@ Execution of the program involves mapping the program's public key to an entrypo
 The interface is best described by the `Instruction::data` that the user encodes.
 
 * `CreateAccount` - This allows the user to create an account with an allocated data array and assign it to a Program.
+* `CreateAccountWithSeed` - Same as `CreateAccount`, but the new account's address is derived from 
+  -  the funding account's pubkey,
+  -  a mnemonic string (seed), and
+  -  the pubkey of the Program
 * `Assign` - Allows the user to assign an existing account to a program.
 * `Transfer` - Transfers lamports between accounts.
 
@@ -51,6 +55,7 @@ To pass messages between programs, the receiving program must accept the message
 
 * There is no dynamic memory allocation. Client's need to use `CreateAccount` instructions to create memory before passing it to another program. This instruction can be composed into a single transaction with the call to the program itself.
 * `CreateAccount` and `Assign` guarantee that when account is assigned to the program, the Account's data is zero initialized.
+* Transactions that assign an account to a program or allocate space must be signed by the Account address' private key unless the Account is being created by `CreateAccountWithSeed`, in which case there is no corresponding private key for the account's address/pubkey.
 * Once assigned to program an Account cannot be reassigned.
 * Runtime guarantees that a program's code is the only code that can modify Account data that the Account is assigned to.
 * Runtime guarantees that the program can only spend lamports that are in accounts that are assigned to it.