added docs for transaction_ignore_urls (#1086)

Author: beniwohli

docs/configuration.asciidoc

@@ -419,6 +419,20 @@ A list of exception types to be filtered.
 Exceptions of these types will not be sent to the APM Server.
 
 
+[float]
+[[config-transaction-ignore-urls]]
+==== `transaction_ignore_urls`
+[options="header"]
+|============
+| Environment                           | Django/Flask              | Default  | Example
+| `ELASTIC_APM_TRANSACTION_IGNORE_URLS` | `TRANSACTION_IGNORE_URLS` | `[]`     | `['/api/ping', '/static/*']`
+| multiple values separated by commas, without spaces |||
+|============
+
+A list of URLs for which the agent should not capture any transaction data.
+
+Optionally, `*` can be used to match multiple URLs at once.
+
 [float]
 [[config-transactions-ignore-patterns]]
 ==== `transactions_ignore_patterns`
@@ -430,8 +444,11 @@ Exceptions of these types will not be sent to the APM Server.
 |============
 
 A list of regular expressions.
-Transactions that match any of the of the configured patterns will be ignored and not sent to the APM Server.
+Transactions with a name that matches any of the configured patterns will be ignored and not sent to the APM Server.
 
+NOTE: as the the name of the transaction can only be determined at the end of the transaction,
+the agent might still cause overhead for transactions ignored through this setting.
+If agent overhead is a concern, we recommend <<config-transaction-ignore-urls,`transaction_ignore_urls`>> instead.
 
 [float]
 [[config-server-timeout]]