Add futureSprint() function in the documentation

The futureSprint() function is not added yet in the documentation.

• https://confluence.atlassian.com/jirasoftwarecloud/advanced-searching-functions-reference-764478342.html

Also, suggestion to update documentation about openSprint() function. See below customer's suggestion:

please also make the docs for "openSprints()" more clear: instead of writing "Search for issues that are assigned to a Sprint that has not yet been completed", I suggest you write "Search for issues that are assigned to a Sprint that was started, but has not yet been completed." or something similar.