API ref: Improve clientKey parameter across API refference

[drobnikj]

There is clientKey query paramater in different request queue API endpoint.
The parameter is same and we just copy & paste it across endpoints.

• It would be nice to have it defined in one place.
• We need to rethink options style: form(better to use simple as value) and explode: true(not necessary)

Originally posted by @janbuchar in #1591 (comment)