feat(libra): guide expense description toward categorization

Expand the expense Description helper text to prompt for a detailed
description (what the purchase/invoice/bill was + what it was used for:
event/project/etc) and point users to the "Other" account when none
of the existing accounts match.

Client feedback: with no fixed category taxonomy yet, a richer
description lets us reconcile/recategorize later even when the expense
lands on the Other account.

Refs aiolabs/webapp#137

src/modules/expenses/components/AddExpense.vue

@@ -109,7 +109,9 @@
                 />
               </FormControl>
               <FormDescription>
-                Describe what this expense was for
+                Use the "Other" account if you don't see one that matches. Add a
+                detailed description of the purchase / invoice / bill, and what
+                it was used for (event / project / etc).
               </FormDescription>
               <FormMessage />
             </FormItem>