;doc: update CLI usage texts

hledger/Hledger/Cli/Commands/Accounts.txt

@@ -3,10 +3,16 @@ Show account names.
 
 _FLAGS
 
-This command lists account names, either declared with account
-directives (--declared), posted to (--used), or both (the default). With
-query arguments, only matched account names and account names referenced
-by matched postings are shown.
+This command lists account names. By default it shows all known
+accounts, either used in transactions or declared with account
+directives.
+
+With query arguments, only matched account names and account names
+referenced by matched postings are shown.
+
+Or it can show just the used accounts (--used), the declared accounts
+(--declared), the accounts used but not declared (--undeclared), or the
+first account matched by an account name pattern, if any (--find).
 
 It shows a flat list by default. With --tree, it uses indentation to
 show the account hierarchy. In flat mode you can add --drop N to omit
@@ -21,7 +27,14 @@ account's declaration, if any, and the account's overall declaration
 order; these may be useful when troubleshooting account display order.
 
 With --directives, it adds the account keyword, showing valid account
-directives which can be pasted into a journal file.
+directives which can be pasted into a journal file. This is useful
+together with --undeclared to update your account declarations when
+check accounts reports undeclared accounts.
+
+The --find flag can be used to look up a single account name, in the
+same way that the aregister command does. It returns the
+alphanumerically-first matched account name, or if none can be found, it
+fails with a non-zero exit code.
 
 Examples:
 
@@ -34,3 +47,6 @@ expenses:supplies
 income:gifts
 income:salary
 liabilities:debts
+
+$ hledger accounts --undeclared --directives >> $LEDGER_FILE
+$ hledger check accounts