Update actions expression documentation for ternary like behaviour (#24357)

Co-authored-by: Siara <108543037+SiaraMist@users.noreply.github.com>

content/actions/learn-github-actions/expressions.md

@@ -105,6 +105,22 @@ env:
 * {% data variables.product.prodname_dotcom %} ignores case when comparing strings.
 * Objects and arrays are only considered equal when they are the same instance.
 
+{% data variables.product.prodname_dotcom %} offers ternary operator like behaviour that you can use in expressions. By using a ternary operator in this way, you can dynamically set the value of an environment variable based on a condition, without having to write separate if-else blocks for each possible option.
+
+#### Example
+
+{% raw %}
+
+```yaml
+env:
+  MY_ENV_VAR: ${{ github.ref == 'refs/heads/main' && 'value_for_main_branch' || 'value_for_other_branches' }}
+```
+
+{% endraw %}
+
+In this example, we're using a ternary operator to set the value of the `MY_ENV_VAR` environment variable based on whether the {% data variables.product.prodname_dotcom %} reference is set to `refs/heads/main` or not. If it is, the variable is set to `value_for_main_branch`. Otherwise, it is set to `value_for_other_branches`.
+It is important to note that the first value after the `&&` condition must be `truthy` otherwise the value after the `||` will always be returned. 
+
 ## Functions
 
 {% data variables.product.prodname_dotcom %} offers a set of built-in functions that you can use in expressions. Some functions cast values to a string to perform comparisons. {% data variables.product.prodname_dotcom %} casts data types to a string using these conversions: