docs: add LA note and update app users

Here's the example to give you an idea:

type EntityType int

const (
    Resource EntityType = iota
    DataSource
)

func (et EntityType) String() string {
    return [...]string{"resource", "datasource"}[et]
}

type AvailabilityType int

const (
    Beta AvailabilityType = iota + 1
    Limited
)

type DescriptionBuilder struct {
    entityType       EntityType
    mainText         string
    availabilityType AvailabilityType
}

func (db *DescriptionBuilder) EntityType(entityType EntityType) *DescriptionBuilder {
    db.entityType = entityType
    return db
}

func (db *DescriptionBuilder) MainText(mainText string) *DescriptionBuilder {
    db.mainText = mainText
    return db
}

func (db *DescriptionBuilder) AvailabilityType(availabilityType AvailabilityType) *DescriptionBuilder {
    db.availabilityType = availabilityType
    return db
}

func (db *DescriptionBuilder) Build() string {
    var builder strings.Builder

    if db.mainText != "" {
        builder.WriteString(db.mainText)

        if db.availabilityType != 0 {
            builder.WriteString(" ")
        }
    }

    commonPart := `
**This %[1]s is in the %[2]s stage and may change without notice.** %[3]s
the ` + "`PROVIDER_AIVEN_ENABLE_BETA`" + ` environment variable to use the %[1]s.`

    switch db.availabilityType {
    case Beta:
        builder.WriteString(fmt.Sprintf(
            commonPart,
            db.entityType.String(),
            "beta",
            "Set",
        ))
    case Limited:
        builder.WriteString(fmt.Sprintf(
            commonPart,
            db.entityType.String(),
            "limited availability",
            "\nTo enable this feature, contact the [sales team](mailto:sales@aiven.io). After it's enabled, set",
        ))
    }

    return builder.String()
}

Usage:

new(DescriptionBuilder).
    // Usage of `EntityType` is optional. It defaults to `Resource`,
    // so it could be omitted for them.
    EntityType(DataSource).
    MainText("Main text.").
    AvailabilityType(Beta).
    Build()

I would recommend to either put it in userconfig directly, or create a nested package there, e.g. desc, and put it there.

Edit: never mind, looks like the existing builder is still used somewhere. Let's adjust it, I've prepared a PR to do that: https://github.com/aiven/terraform-provider-aiven/pull/1648. Let's get it merged first, and then we can rebase your branch when it's in the main one.

aiven / terraform-provider-aiven

docs: add LA note and update app users #1639

About this change—what it does