Transactions: a guide in Part II, and reference pages for (std stm)

Part II's section on concurrency has a new subsection, "Shared state:
atomics and transactions": when to share data instead of sending it,
locations and atomically, the rules for a transaction function, waiting
with tx-retry, a transaction as an event in a choose, tx-or-else, the
queue, map and counter, and what transactions cost. The examples use
the card game and were compiled and run.

Part III has a section on std/stm that links the four reference pages:
(std stm), and (std stm queue), (std stm map) and (std stm counter) in
a directory of their own, as (text xml) has.

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modules/std/stm/index.sz

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+(meta (title "std stm: transactional data structures")
+      (sort title))