Update sysvar docs (#17493)

docs/src/developing/programming-model/accounts.md

@@ -106,11 +106,14 @@ To check an account's validity, the program should either check the account's
 address against a known value or check that the account is indeed owned
 correctly (usually owned by the program itself).
 
-One example is when programs read a sysvar. Unless the program checks the
+One example is when programs use a sysvar account. Unless the program checks the
-address or owner, it's impossible to be sure whether it's a real and valid
+account's address or owner, it's impossible to be sure whether it's a real and
-sysvar merely by successful deserialization. Accordingly, the Solana SDK [checks
+valid sysvar account merely by successful deserialization of the account's data.
-the sysvar's validity during
+Accordingly, the Solana SDK [checks the sysvar account's validity during
 deserialization](https://github.com/solana-labs/solana/blob/a95675a7ce1651f7b59443eb146b356bc4b3f374/sdk/program/src/sysvar/mod.rs#L65).
+A alternative and safer way to read a sysvar is via the sysvar's [`get()`
+function](https://github.com/solana-labs/solana/blob/64bfc14a75671e4ec3fe969ded01a599645080eb/sdk/program/src/sysvar/mod.rs#L73)
+which doesn't require these checks.
 
 If the program always modifies the account in question, the address/owner check
 isn't required because modifying an unowned (could be the malicious account with

docs/src/developing/runtime-facilities/sysvars.md

@@ -9,11 +9,24 @@ known addresses published along with the account layouts in the
 crate](https://docs.rs/solana-program/VERSION_FOR_DOCS_RS/solana_program/sysvar/index.html),
 and outlined below.
 
-To include sysvar data in program operations, pass the sysvar account address in
+There are two ways for a program to access a sysvar.
-the list of accounts in a transaction. The account can be read in your
+
-instruction processor like any other account. Access to sysvars accounts is
+The first is to query the sysvar at runtime via the sysvar's `get()` function:
+
+```
+let clock = Clock::get()
+```
+
+The second is to pass the sysvar to the program as an account by including its address as one of the accounts in the `Instruction` and then deserializing the data during execution.  Access to sysvars accounts is
 always _readonly_.
 
+```
+let clock_sysvar_info = next_account_info(account_info_iter)?;
+let clock = Clock::from_account_info(&clock_sysvar_info)?;
+```
+
+The first method is more efficient and does not require that the sysvar account be passed to the program, or specified in the `Instruction` the program is processing.
+
 ## Clock
 
 The Clock sysvar contains data on cluster time, including the current slot,

docs/src/terminology.md

@@ -255,7 +255,7 @@ Tokens forfeit to the [cluster](terminology.md#cluster) if malicious [validator]
 
 ## sysvar
 
-A synthetic [account](terminology.md#account) provided by the runtime to allow programs to access network state such as current tick height, rewards [points](terminology.md#point) values, etc.
+A synthetic [account](terminology.md#account) maintained by the runtime and provided to programs which provide cluster state such as current tick height, rewards [points](terminology.md#point) values, etc.  Sysvars can either be [passed to the program as an account or queried by the program via a syscall](developing/runtime-facilities/sysvars.md).
 
 ## thin client