Revert "Revert "clarify effect of should command on implicit DOM asse…

…rtions (#1289)""

This reverts commit 296da56.

source/api/commands/should.md

@@ -349,6 +349,10 @@ cy.get('button').click()
 
 # Notes
 
+## Effect on default DOM assertions
+
+When you chain `.should()` on a DOM-based command, the default `.should('exist')` assertion is skipped. This may result in an unexpected behavior such as negative assertions passing even when the element doesn't exist in the DOM. See {% url 'Default Assertions' introduction-to-cypress#Default-Assertions %} for more.
+
 ## Subjects
 
 ### How do I know which assertions change the subject and which keep it the same?

source/guides/core-concepts/introduction-to-cypress.md

@@ -674,10 +674,30 @@ Even more - action commands will automatically wait for their element to reach a
 {% note success Core Concept %}
 All DOM based commands automatically wait for their elements to exist in the DOM.
 
-You **never** need to write {% url "`.should('exist')`" should %} after a DOM based command.
+You don't need to write {% url "`.should('exist')`" should %} after a DOM based command, unless you chain extra `.should()` assertions.
 {% endnote %}
 
-These rules are pretty intuitive, and most commands give you flexibility to override or bypass the default ways they can fail, typically by passing a `{force: true}` option.
+{% note danger "Negative DOM assertions" %}
+If you chain any `.should()` command, the default `.should('exist')` is not asserted. This does not matter for most *positive* assertions, such as `.should('have.class')`, because those imply existence in the first place, but if you chain *negative* assertions ,such as `.should('not.have.class')`, they will pass even if the DOM element doesn't exist:
+
+```
+cy.get('.does-not-exist').should('not.be.visible')         // passes
+cy.get('.does-not-exist').should('not.have.descendants')   // passes
+```
+
+This also applies to custom assertions such as when passing a callback:
+
+```
+// passes, provided the callback itself passes
+cy.get('.does-not-exist').should(($element) => {   
+  expect($element.find('input')).to.not.exist 
+})
+```
+
+There's an {% url 'open discussion' https://github.com/cypress-io/cypress/issues/205 %} about this behavior.
+{% endnote %}
+
+These rules are pretty intuitive, and most commands give you the flexibility to override or bypass the default ways they can fail, typically by passing a `{force: true}` option.
 
 ### Example #1: Existence and Actionability