Add documentation about multiple values of the search filter

[alanpoulain]

Searching multiple values for the same property is not documented.
Example:
/books?author[]=/authors/1&author[]=/authors/2

See: api-platform/core#1985

[teohhanhui]

Definitely my fault. 🙈

[lcottingham]

How does this work on graphql when the schema enforces a property: String during introspection? How would I input multiple search values for the same property?

For example, variables as {foo: ['bar', 'baz']} is invalid. Unsure how to feed multiple values into the query without angering the IDE.

query getObjects ($values: String) { objects (foo: $values) { edges { ...