Update INVARIANT_CHECKS documentation in stellar-core_example.cfg (#4598

)

# Description

Resolves #4596

<!---

Describe what this pull request does, which issue it's resolving
(usually applicable for code changes).

--->

# Checklist
- [x] Reviewed the
[contributing](https://github.com/stellar/stellar-core/blob/master/CONTRIBUTING.md#submitting-changes)
document
- [x] Rebased on top of master (no merge commits)
- [x] Ran `clang-format` v8.0.0 (via `make format` or the Visual Studio
extension)
- [x] Compiles
- [x] Ran all tests
- [ ] If change impacts performance, include supporting evidence per the
[performance
document](https://github.com/stellar/stellar-core/blob/master/performance-eval/performance-eval.md)

docs/stellar-core_example.cfg

@@ -425,6 +425,8 @@ RUN_STANDALONE=false
 # on bucket apply.
 # Strings specified are matched (as regex) against the list of invariants.
 # For example, to enable all invariants use ".*"
+# INVARIANT_CHECKS = [.*]
+#
 # List of invariants:
 # - "AccountSubEntriesCountIsValid"
 #     Setting this will cause additional work on each operation apply - it
@@ -440,7 +442,8 @@ RUN_STANDALONE=false
 #     detailed information about what is checked see the comment in the header
 #     invariant/BucketListIsConsistentWithDatabase.h.
 #     The overhead may cause a system to catch-up more than once before being
-#     in sync with the network.
+#     in sync with the network. This may also increase startup time as it checks 
+#     the entire state of the database.
 # - "CacheIsConsistentWithDatabase"
 #     Setting this will cause additional work on each operation apply - it
 #     checks if internal cache of ledger entries is consistent with content of
@@ -466,7 +469,12 @@ RUN_STANDALONE=false
 #     in the header invariant/LiabilitiesMatchOffers.h.
 #     The overhead may cause slower systems to not perform as fast as the rest
 #     of the network, caution is advised when using this.
-INVARIANT_CHECKS = []
+INVARIANT_CHECKS = [ "AccountSubEntriesCountIsValid",
+"ConservationOfLumens",
+"ConstantProductInvariant",
+"LedgerEntryIsValid",
+"LiabilitiesMatchOffers",
+"SponsorshipCountIsValid" ]
 
 
 # MANUAL_CLOSE (true or false) defaults to false