Document new search API

[jvillafanez]

Most of the information should be in owncloud/core#31873

owncloud/core#25373 (comment) also contains some interesting information about how to check the available reports for a specific folder.

There is also a capability entry in the OCS capabilities endpoint for this.

[PVince81]

@settermjd please let us know if that's enough information to start

[PVince81]

Estimate 1md or more depending how difficult it is to add code examples / tables ?

[settermjd]

Am on it, @PVince81.

[PVince81]

moving to next sprint as we're tight on time

[settermjd]

Closing, as this was implemented in #224.