docs: Add documentation for using child/dual commands in TS

Author: xumepadismal

content/guides/tooling/typescript-support.md

@@ -120,6 +120,14 @@ by any users of your custom command.
 
 </Alert>
 
+<Alert type="info">
+
+Please note, types of all the parameters taken by implementation callback are
+inferred automatically based on declared interface. Thus, in the example above
+the `value` will be of type `string` implicitly.
+
+</Alert>
+
 In your specs, you can now use the custom command as expected
 
 ```typescript
@@ -132,6 +140,77 @@ it('works', () => {
 })
 ```
 
+#### Adding child or dual commands
+
+When you add a custom command with `prevSubject` Cypress will infer subject type
+automatically based on kind of specified `prevSubject`(s)
+
+```typescript
+// in cypress/support/index.ts
+// load type definitions that come with Cypress module
+/// <reference types="cypress" />
+
+declare global {
+  namespace Cypress {
+    interface Chainable {
+      /**
+       * Custom command to type few random words into input elements
+       * @param count=3
+       * @example cy.get('input').typeRandomWords()
+       */
+      typeRandomWords(count?: number, options?: Partial<TypeOptions>): Chainable<Element>
+    }
+  } 
+}
+```
+
+```typescript
+// cypress/support/index.ts
+Cypress.Commands.add('typeRandomWords', {prevSubject: 'element'}, (
+    subject /* :JQuery<HTMLElement> */, count = 3, options?,
+) => {
+  return cy.wrap(subject).type(generateRandomWords(cound), options)
+})
+```
+
+
+#### Overwriting child or dual commands
+
+When overwriting either built-in or custom commands which make use of `prevSubject`
+you have to help the type-checker to understand this.
+
+```typescript
+interface TypeOptions extends Cypress.TypeOptions {
+  sensitive: boolean
+}
+
+Cypress.Commands.overwrite<'type', 'element'>('type', (
+    originalFn, element, text, options?: Partial<TypeOptions>,
+) => {
+  if (options && options.sensitive) {
+    // turn off original log
+    options.log = false
+    // create our own log with masked message
+    Cypress.log({
+      $el: element,
+      name: 'type',
+      message: '*'.repeat(text.length),
+    })
+  }
+  
+  return originalFn(element, text, options)
+})
+```
+
+As you can see there are generic parameters `<'type', 'element'>` are used:
+
+1. This is the command name, should equal to actual first parameter passed to `cy.overwite`
+2. Type of prevSubject that is used by the original command. Possible values:
+   - 'element' will infer second param as `JQuery<HTMLElement>`
+   - 'window' will infer second param as `Window`
+   - 'document' will infer second param as `Document`
+   - 'optional' will infer second param as `unknown`
+
 #### Examples:
 
 - Find