Document postBalance field (#12628)

(cherry picked from commit e03a64ae1b)

Co-authored-by: Michael Vines <mvines@gmail.com>

docs/src/apps/jsonrpc-api.md

@@ -339,6 +339,7 @@ The result field will be an object with the following fields:
   - `rewards: <array>` - an array of JSON objects containing:
     - `pubkey: <string>` - The public key, as base-58 encoded string, of the account that received the reward
     - `lamports: <i64>`- number of reward lamports credited or debited by the account, as a i64
+    - `postBalance: <u64>` - account balance in lamports after the reward was applied
   - `blockTime: <i64 | null>` - estimated production time, as Unix timestamp (seconds since the Unix epoch). null if not available
 
 #### Example: