Improve documentation of the cross function

The documentation of the cross function should be reviewed and improved:

• make sure the example expressions are correct (currently cell.cross("People","Name").cells["Address"].value[0], perhaps should be cell.cross("People","Name").cells["Address"][0].value instead?)
• document the use of the cross function to access any cell of the current project (see this comment)
• maybe more of the content from the wiki could be integrated in the docs directly

Start with the linked GREL cross-function documentation and verify the example expressions against the function behavior. Read the referenced comment in issue 3305 and compare the wiki's “Combining datasets” recipe. Done means corrected examples, documented access to cells in the current project, and any useful wiki content integrated into the documentation.