Update bulk pricing tiers documentation to cover null case

[IamDavidovich]

Update schemas for pricelist bulk tiers to cover that null may be returned when there is no upper limit on the quantity for a given tier.

What changed?

• Update schemas for pricelist bulk tiers to cover that null may be returned when there is no upper limit on the quantity for a given tier.
• Unrelatedly, add a ' to "this tiers pricing" since it's a possessive rather than a plural.

Release notes draft

• Clarified documentation for pricelist bulk tier pricing to explain tiers with no upper quantity limit.

Anything else?

Nope 😄

[IamDavidovich]

@bigcommerce/dev-docs-collaborators Can I get some 👀 on this when you have time?

[becomevocal]

Of course @IamDavidovich!