adds documentation about how to group and order inputs to a union

[j23schoen]

Issues about this here:
knex/knex#739
knex/knex#913

Here's the PR that addressed it:
knex/knex#5030

This documentation should make it clear how to apply the following clauses to inputs of the union:

• group
• having
• order
• limit
• offset

I'm open to some wording adjustments to fit better with the theme of the knex docs, but I hope I've conveyed the issue and solution

[CodingItWrong]

I'd find this change helpful. Without this I would assume I can just chain order, limit etc onto the previous statements as shown in the documentation. Would save me a lot of confusion.

[OlivierCavadenti]

Thanks !