http: doc: clarify when/who cleans up a sol_http_client_connection

[cmarcelo]

The function sol_http_client_request() returns a sol_http_client_connection* but doesn't document whether we need to care about cleaning up, or if this is handled by Soletta itself.

http://solettaproject.github.io/docs/c-api/group__HTTP__CLIENT.html#ga8cd4dc21acf75f83d251cf4865961e2a

It seems we need to care about cleaning them only if the callback was not called yet.