Add recommendations on non-idempotent operations to vshard docs

[xuniq]

**Related issue: #5202

Product: Tarantool (vshard module)
Branches: 2.11, latest?
Root document: vshard docs

• https://www.tarantool.io/ru/doc/2.11/book/admin/vshard_admin/#adding-data
• https://www.tarantool.io/ru/doc/2.11/reference/reference_rock/vshard/vshard_router/#lua-function.vshard.router.callrw
• https://www.tarantool.io/ru/doc/2.11/reference/reference_rock/vshard/vshard_router/#lua-function.vshard.router.call

SME: @ sergepetrenko @ a1div0

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