Improve documentation for from_env and from_zk

- Add a new paragraph to make it possible to find
- Add documentation for default values when using these attributes

docs/en/operations/configuration-files.md

@@ -67,6 +67,8 @@ generates merged configuration file:
 </clickhouse>
 ```
 
+### Using from_env and from_zk
+
 To specify that a value of an element should be replaced by the value of an environment variable, you can use attribute `from_env`.
 
 Example with `$MAX_QUERY_SIZE = 150000`:
@@ -93,6 +95,59 @@ which is equal to
 </clickhouse>
 ```
 
+The same is possible using `from_zk`:
+
+``` xml
+<clickhouse>
+    <postgresql_port from_zk="/zk_configs/postgresql_port"/>
+</clickhouse>
+```
+
+```
+# clickhouse-keeper-client
+/ :) touch /zk_configs
+/ :) create /zk_configs/postgresql_port "9005"
+/ :) get /zk_configs/postgresql_port
+9005
+```
+
+which is equal to
+
+
+``` xml
+<clickhouse>
+    <postgresql_port>9005</postgresql_port>
+</clickhouse>
+```
+
+#### Default values for from_env and from_zk attributes
+
+It's possible to set the default value and substitute it only if the environment variable or zookeeper node is set using `replace="1"`.
+
+With previous example, but `MAX_QUERY_SIZE` is unset:
+
+``` xml
+<clickhouse>
+    <profiles>
+        <default>
+            <max_query_size from_env="MAX_QUERY_SIZE" replace="1">150000</max_query_size>
+        </default>
+    </profiles>
+</clickhouse>
+```
+
+will take the default value
+
+``` xml
+<clickhouse>
+    <profiles>
+        <default>
+            <max_query_size>150000</max_query_size>
+        </default>
+    </profiles>
+</clickhouse>
+```
+
 ## Substituting Configuration {#substitution}
 
 The config can define substitutions. There are two types of substitutions: