Update Y error bars help page to also fit X error bars

Change-Id: I56fd5e196e84b97ad5505a60fda0260221aa0a9f
Reviewed-on: https://gerrit.libreoffice.org/7139
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
Tested-by: Markus Mohrhard <markus.mohrhard@googlemail.com>