docs: added info about new status codes

[frolvanya]

This PR is adding an information about new possible status codes that can be returned by jsonrpc

From now on, jsonrpc will start returning other HTTP codes than 200 OK for some errors:

• On internal server error it will return 500
• On timeout error it will return 408
• On request validation error will return 400

Changes were introduced in https://github.com/near/nearcore/pull/11806 and https://github.com/near/nearcore/pull/11822

[frolvanya]

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