Summer 19 is 1 week away, but that won't stop you from preparing it beforehand in your Summer 19 Sandbox or Scratch org.

lightning:empApi does not show LWC button as enabled as its referring to the Spring version of docs.

Also , the release notes dont tell us about how to use empAPI in LWC. I could not find any public documentation on about how to use it.

A quick way to refer docs of lwc is Summer 19 org is to visit the url

https://yourSummer19org.salesforce.com/docs/component-library/bundle/lightning-emp-api/documentation

Sample Example :

<template>
    <lightning-card title="EmpApi Example" icon-name="custom:custom14">
        <div class="slds-m-around_medium">
            <p>Use the buttons below to subscribe and unsubscribe to a streaming channel!</p>
            <lightning-input label="Channel Name" value={channelName}
                onchange={handleChannelName}></lightning-input>
            <lightning-button variant="success" label="Subscribe" title="Subscribe"
                onclick={handleSubscribe} disabled={isSubscribeDisabled}
                class="slds-m-left_x-small"></lightning-button>
            <lightning-button variant="destructive" label="Unsubscribe" title="Unsubscribe"
                onclick={handleUnsubscribe} disabled={isUnsubscribeDisabled}
                class="slds-m-left_x-small"></lightning-button>
        </div>
    </lightning-card>
</template>

JS:

import { LightningElement, track } from 'lwc';
import { subscribe, unsubscribe, onError, setDebugFlag, isEmpEnabled } from 'lightning/empApi';

export default class EmpApiLWC extends LightningElement {
    @track channelName = '/event/Test__e';
    @track isSubscribeDisabled = false;
    @track isUnsubscribeDisabled = !this.isSubscribeDisabled;

    subscription = {};

    // Tracks changes to channelName text field
    handleChannelName(event) {
        this.channelName = event.target.value;
    }

    // Handles subscribe button click
    handleSubscribe() {
        // Callback invoked whenever a new event message is received
        const messageCallback = function(response) {
            console.log('New message received : ', JSON.stringify(response));
            // Response contains the payload of the new message received
        };

        // Invoke subscribe method of empApi. Pass reference to messageCallback
        subscribe(this.channelName, -1, messageCallback).then(response => {
            // Response contains the subscription information on successful subscribe call
            console.log('Successfully subscribed to : ', JSON.stringify(response.channel));
            this.subscription = response;
            this.toggleSubscribeButton(true);
        });
    }

    // Handles unsubscribe button click
    handleUnsubscribe() {
        this.toggleSubscribeButton(false);

        // Invoke unsubscribe method of empApi
        unsubscribe(this.subscription, response => {
            console.log('unsubscribe() response: ', JSON.stringify(response));
            // Response is true for successful unsubscribe
        });
    }

    toggleSubscribeButton(enableSubscribe) {
        this.isSubscribeDisabled = enableSubscribe;
        this.isUnsubscribeDisabled = !enableSubscribe;
    }

    registerErrorListener() {
        // Invoke onError empApi method
        onError(error => {
            console.log('Received error from server: ', JSON.stringify(error));
            // Error contains the server-side error
        });
    }
}

As the documentation says, you don't need to worry about those particular limits. Those are for Standard-Volume Platform Events, and you can't create those anymore. All new platform events are high volume by default.

The difference between the publish and delivery is the actor of the action. For example: if a user updates a record's field to a certain value, you want the object's trigger to publish a new event that will be delivered to another implementation. The trigger publishes the event. The platform delivers the event to its subscribers.