docs: Document Cypress.Commands.addAll() (#4414)

Co-authored-by: Emily Rohrbough <[email protected]>

content/api/cypress-api/custom-commands.md

@@ -21,6 +21,8 @@ evaluated via an import statement in your
 ```javascript
 Cypress.Commands.add(name, callbackFn)
 Cypress.Commands.add(name, options, callbackFn)
+Cypress.Commands.addAll(callbackObj)
+Cypress.Commands.addAll(options, callbackObj)
 Cypress.Commands.overwrite(name, callbackFn)
 ```
 
@@ -30,6 +32,10 @@ Cypress.Commands.overwrite(name, callbackFn)
 
 ```javascript
 Cypress.Commands.add('login', (email, pw) => {})
+Cypress.Commands.addAll({
+  login(email, pw) {},
+  visit(orig, url, options) {},
+})
 Cypress.Commands.overwrite('visit', (orig, url, options) => {})
 ```
 
@@ -43,6 +49,10 @@ The name of the command you're either adding or overwriting.
 
 Pass a function that receives the arguments passed to the command.
 
+**<Icon name="angle-right"></Icon> callbackObj** **_(Object)_**
+
+An object with `callbackFn`s as properties.
+
 **<Icon name="angle-right"></Icon> options** **_(Object)_**
 
 Pass in an options object to define the implicit behavior of the custom command.