mempool: Use m_limit for UpdateTransactionsFromBlock

Since:

- UpdateTransactionsFromBlock is only called by
  MaybeUpdateMempoolForReorg, which calls it with the gArgs-determined
  ancestor limits
- UpdateForDescendants is only called by UpdateTransactionsFromBlock
  with the ancestor limits unchanged

We can remove the requirement to specify the ancestor limits for both
UpdateTransactionsFromBlock and UpdateForDescendants and just use the
values in the m_limits member.

Also move some removed comments to MemPoolLimits struct members.

The uint64_t cast in UpdateForDescendants is not new behavior,
see the diff in CChainState::MaybeUpdateMempoolForReorg for where they
were previously.

src/kernel/mempool_limits.h

@@ -16,9 +16,13 @@ namespace kernel {
  * Most of the time, this struct should be referenced as CTxMemPool::Limits.
  */
 struct MemPoolLimits {
+    //! The maximum allowed number of transactions in a package including the entry and its ancestors.
     int64_t ancestor_count{DEFAULT_ANCESTOR_LIMIT};
+    //! The maximum allowed size in virtual bytes of an entry and its ancestors within a package.
     int64_t ancestor_size_vbytes{DEFAULT_ANCESTOR_SIZE_LIMIT_KVB * 1'000};
+    //! The maximum allowed number of transactions in a package including the entry and its descendants.
     int64_t descendant_count{DEFAULT_DESCENDANT_LIMIT};
+    //! The maximum allowed size in virtual bytes of an entry and its descendants within a package.
     int64_t descendant_size_vbytes{DEFAULT_DESCENDANT_SIZE_LIMIT_KVB * 1'000};
 };
 } // namespace kernel