Merge pull request #3114 from cypress-io/warn-about-env

docs: add notice about diff b/t Cypress.env and real env vars

source/_partial/cypress_env_var_warning.md

@@ -0,0 +1,3 @@
+{% note warning "Difference between OS-level and Cypress environment variables" %}
+In Cypress, "environment variables" are variables that are accessible via `Cypress.env`. These are not the same as OS-level environment variables. However, [it is possible to set Cypress environment variables from OS-level environment variables](/guides/guides/environment-variables.html#Option-3-CYPRESS).
+{% endnote %}

source/api/cypress-api/env.md

@@ -14,6 +14,8 @@ Environment variables set using `Cypress.env` _are only in scope for the current
 Cypress runs each spec file in isolation: the browser is exited between specs. Environment variables added or changed in one spec won't be visible in other specs.
 {% endnote %}
 
+{% partial cypress_env_var_warning %}
+
 # Syntax
 
 ```javascript

source/guides/guides/environment-variables.md

@@ -2,6 +2,8 @@
 title: Environment Variables
 ---
 
+{% partial cypress_env_var_warning %}
+
 Environment variables are useful when:
 
 - Values are different across developer machines.
@@ -123,7 +125,7 @@ Cypress.env('api_server') // 'http://localhost:8888/api/v1/'
 
 ## Option #3: `CYPRESS_*`
 
-Any environment variable on your machine that starts with either `CYPRESS_` or `cypress_` will automatically be added and made available to you.
+Any OS-level environment variable on your machine that starts with either `CYPRESS_` or `cypress_` will automatically be added to Cypress' environment variables and made available to you.
 
 Conflicting values will override values from your configuration file (`cypress.json` by default) and `cypress.env.json` files.