[docs] Align docs with EULA (source of truth)

[oliviertassinari]

Fix the inconsistency reported in #1961

• docs: can use XGrid for development
• license warning: can't use XGrid fo development
• EULA: can't use XGrid for development

This is meant to match the policy of the existing commercial alternative solutions. We don't need to differentiate on this end. If we want to change the policy in the future, then we need to update it all at once.