;doc: update CLI usage texts

hledger/Hledger/Cli/Commands/Balance.txt

@@ -242,6 +242,20 @@ in the terminal. Here are some ways to handle that:
 -   Output as HTML and view with a browser:
     hledger bal -D -o a.html && open a.html
 
+Showing declared accounts
+
+With --declared, accounts which have been declared with an account
+directive will be included in the balance report, even if they have no
+transactions. (Since they will have a zero balance, you will also need
+-E/--empty to see them.)
+
+More precisely, leaf declared accounts (with no subaccounts) will be
+included, since those are usually the more useful in reports.
+
+The idea of this is to be able to see a useful "complete" balance
+report, even when you don't have transactions in all of your declared
+accounts yet.
+
 Commodity layout
 
 With --layout, you can control how amounts with more than one commodity