Update API docs for endpoints with signature query parameter

[Pijukatel]

These endpoints can have an optional query parameter signature:
https://docs.apify.com/api/v2/key-value-store-record-get
https://docs.apify.com/api/v2/key-value-store-keys-get
https://docs.apify.com/api/v2/dataset-items-get

Add it to the documentation.

(Discussed in apify/apify-client-python#497)