Document cross-engine transactions

Related dev. issue(s): https://github.com/tarantool/tarantool/issues/1803
Product: Tarantool
Since: 3.4
Root document: https://www.tarantool.io/en/doc/latest/platform/atomic/txn_mode_mvcc/#txn-mode-transaction-manager
SME: @ locker

Details

Scope:

• Remove the note "Currently, you cannot use several different storage engines within one transaction"
• Add the example and the description of the updates to "Examples with MVCC enabled and disabled"

Tarantool now supports mixing statements for different storage engines
in the same transaction, for example:

local memtx = box.schema.space.create('memtx', {engine = 'memtx'})
memtx:create_index('primary')
local vinyl = box.schema.space.create('vinyl', {engine = 'vinyl'})
vinyl:create_index('primary')

memtx:insert({1, 'a'})
vinyl:insert({2, 'b'})

box.begin()
memtx:replace(vinyl:get(2))
vinyl:replace(memtx:get(1))
box.commit()

Note, since accessing a vinyl space may trigger a fiber yield (to read
a file from the disk), MVCC must be enabled in memtx to make use of
the new feature:

box.cfg{memtx_use_mvcc_engine = true}

Requested by @ locker in https://github.com/tarantool/tarantool/commit/b9c5a1d9641081bca681e2d82fea96dc9398fdfa.

Start with the root document's transaction-manager section and the “Examples with MVCC enabled and disabled” section. Remove the outdated restriction, add the cross-engine transaction example, and document that memtx MVCC must be enabled when accessing vinyl; done means both sections describe the supported behavior accurately.