Add FAQ comment about using expressions in portfolio entries

Author: simon-anz

docs/content/faq.md

@@ -165,6 +165,8 @@ draft: false
 
   Press <kbd>e</kbd> on the highlighted coin to enter holdings and add to your portfolio.
 
+  This dialog supports basic expressions including `+` `-` `*` etc.
+
 ## How do I edit the holdings of a coin in my portfolio?
 
   Press <kbd>e</kbd> on the highlighted coin to edit the holdings.