Fix text formatting and code examples according to Lua style guide

[NickVolynkin]

Resolve #2236

[patiencedaur]

@Totktonada
The names of the new and old objects are the same now, so it's a little confusing. The function returns a totally new object, not the altered old one—is that correct?

[patiencedaur]

Reply from @ Totktonada:
Да, :merge() возвращает новый объект.

[patiencedaur]

@Totktonada
"возвращает определение ключа по полям fieldno = 3 и fieldno = 1"? "для полей"? "содержащее поля"?

[patiencedaur]

Explanation from @ Totktonada:

Давай я в целом про таплы, ключи и определения ключей напишу — должно стать понятней.

Тапл — это массив, строка в спейсе (можно сказать, в таблице, но таплы могут различаться по длине). Возьмем для примера [1, 'foo', 'bar'].

Определение ключа (key definition, key def) — это информация в духе «третье и первое поля тапла» (плюс еще типы в этих полях и еще разные детали). Порядок имеет значение.

Ключ или значение ключа — это в данном случае, собственно, массив из значения третьего и первого поля. Для тапла [1, 'foo', 'bar'] это будет ['bar', 1].

Illustration

[patiencedaur]

@Totktonada
Must all parts have the fieldno and type attributes? Or can there be only one part with those attributes, while the other parts don't have those attributes? It is not quite clear.

[Totktonada]

All parts must have fieldno or field attribute (see #962). All parts must have type attribute. Other attributes are optional. The parts array should have at least one item.

[patiencedaur]

@Totktonada

Lines 11–12:
The definition is usually used in conjunction with an index definition
to extract or compare the index key values.

What does "index key values" mean here? How are index and key related in this context?
I understand that an index is a data structure, while a key is an array.

[Totktonada]

Say, we have a space (named 'users'):

id	email	name
1	foo@bar.baz	Ivan Petrov
2	baz@foo.bar	Petr Ivanov

With primary index by the 'id' field and secondary index by 'email' and 'name' fields.

A get request by the primary index could be box.space.users:get({1}) — here {1} is the key that corresponds the 'id field. If we want to perform a get request using the secondary index, we should pass a key that corresponds its fields: box.space.users:get({'foo@bar.baz', 'Ivan Petrov'}).

So index is usually constructed on a subset of tuple's fields and phrases like 'index key' mean a key that is suitable to perfrom a request using this index.