doc: add signatures for the error types

Author: tejasbadadare

target_chains/ethereum/pulse_sdk/solidity/SchedulerErrors.sol

@@ -4,37 +4,56 @@ pragma solidity ^0.8.0;
 
 library SchedulerErrors {
     // Authorization errors
+
+    /// 0x82b42900
     error Unauthorized();
 
     // Subscription state errors
+
+    /// 0xe7262b66
     error InactiveSubscription();
+    /// 0xf4d678b8
     error InsufficientBalance();
+    /// 0xf6181305
     error CannotUpdatePermanentSubscription();
 
     // Price feed errors
+    /// 0xae2eaaa9
     error InvalidPriceId(bytes32 providedPriceId, bytes32 expectedPriceId);
+    /// 0xf14f93d1
     error InvalidPriceIdsLength(uint256 providedLength, uint256 expectedLength);
+    /// 0x94ec8d9a
     error EmptyPriceIds();
+    /// 0xb3d1acf6
     error TooManyPriceIds(uint256 provided, uint256 maximum);
+    /// 0xe3509591
     error DuplicatePriceId(bytes32 priceId);
+    /// 0xe56ccfaa
     error PriceSlotMismatch();
 
     // Update criteria errors
+    /// 0xa7bcd3ae
     error InvalidUpdateCriteria();
+    /// 0x7e8b0263
     error UpdateConditionsNotMet();
+    /// 0x38fdebae
     error TimestampTooOld(
         uint256 providedUpdateTimestamp,
         uint256 currentTimestamp
     );
+    /// 0x06daa54d
     error TimestampOlderThanLastUpdate(
         uint256 providedUpdateTimestamp,
         uint256 lastUpdatedAt
     );
 
     // Whitelist errors
+    /// 0xbe4b60f7
     error TooManyWhitelistedReaders(uint256 provided, uint256 maximum);
+    /// 0x9941ad5f
     error DuplicateWhitelistAddress(address addr);
 
     // Payment errors
+    /// 0xec58cd53
     error KeeperPaymentFailed();
 }