Help documentation for your Salesforce implementation is very important. How you will do it depends on your time, your skills and your budget. 

Revisiting this for Lightning and expanding it to on page and full help text. 

Video

00:00 - Introduction

00:30 - Indicators

01:45 - Page Structure

02:27 - Help Text

02:51 - Validation Rules

04:58 - Related Record Component

06:07 - Rich Text Error

06:59 - Alerts

14:23 - Snippets

16:10 - Simple Help Docs

17:32 - Box Files

19:53 - Specific Components

22:35 - Home Page

25:53 - In-App Guidance

27:34 - Winter 21

28:18 - Web Tab

29:28 - Spekit

30:46 - Summary

Validation Options

Alert Options

Help Snippets Options

Link to documents eg in Google Docs, SharePoint, Teams, Quip

Help Documents Options

Not Free

Other

Other Apps not recommended

Also see

Examples of good help docs