documenting the question argument

doc/interactive-commands.md

@@ -38,6 +38,10 @@ Both interfaces may serve at the same time. Both respond to simple text command,
 - `unpostpone`: at a time where `gh-ost` is postponing the [cut-over](cut-over.md) phase, instruct `gh-ost` to stop postponing and proceed immediately to cut-over.
 - `panic`: immediately panic and abort operation
 
+### Querying for data
+
+For commands that accept an argumetn as value, pass `?` (question mark) to _get_ current value rather than _set_ a new one.
+
 ### Examples
 
 While migration is running:
@@ -63,6 +67,11 @@ $ echo "chunk-size=250" | nc -U /tmp/gh-ost.test.sample_data_0.sock
 # Serving on TCP port: 10001
 ```
 
+```shell
+$ echo "chunk-size=?" | nc -U /tmp/gh-ost.test.sample_data_0.sock
+250
+```
+
 ```shell
 $ echo throttle | nc -U /tmp/gh-ost.test.sample_data_0.sock