ui: Document implicit vs explicit transactions on statements page

Piyush Singh commented:

We are planning to filter statements on whether they are part of implicit vs explicit transactions in cockroachdb/cockroach#36009. Our goal is to help users understand when transactional write pipelining might have an impact on observed statement performance. We should include an update to the docs for the statements page to include descriptions of the distinction. This might require a brief overview of transactional pipelining and why the observed performance for write statements in explicit transactions is different from implicit transactions.

We expect that this change will be ready for a 19.1 point release.

cc @Amruta-Ranade @celiala

Jira Issue: DOC-261