Update docs

docs/changelog.md

@@ -6,6 +6,7 @@
 
 - Added `names` option to the `parser` block, which controls how does Prometheus validates
   label names.
+- Added [promql/impossible](checks/promql/impossible.md) check.
 
 ### Fixed
 

docs/checks/promql/impossible.md

@@ -0,0 +1,69 @@
+---
+layout: default
+parent: Checks
+grand_parent: Documentation
+---
+
+# promql/impossible
+
+This check will report PromQL queries that can never return anything.
+
+Example query:
+
+```js
+foo{job="bar"} unless sum(foo)
+```
+
+The right hand side (`unless sum(foo)`) cannot ever match any time series returned by the left hand side
+(`foo{job="bar"}`) because `sum()` strips away all labels. This means that we end up with left hand side
+having at least the `job` label, while the right hand side has no labels, so we end up with:
+
+```js
+foo{job="bar"} unless {}
+```
+
+Both sides can only match if they have the same label set, see [Prometheus docs](https://prometheus.io/docs/prometheus/latest/querying/operators/#vector-matching). A result with `{job="bar"}` will never be matched with empty label set `{}`.
+
+## Configuration
+
+This check doesn't have any configuration options.
+
+## How to enable it
+
+This check is enabled by default.
+
+## How to disable it
+
+You can disable this check globally by adding this config block:
+
+```js
+checks {
+  disabled = ["promql/impossible"]
+}
+```
+
+You can also disable it for all rules inside given file by adding
+a comment anywhere in that file. Example:
+
+```yaml
+# pint file/disable promql/impossible
+```
+
+Or you can disable it per rule by adding a comment to it. Example:
+
+```yaml
+# pint disable promql/impossible
+```
+
+## How to snooze it
+
+You can disable this check until given time by adding a comment to it. Example:
+
+```yaml
+# pint snooze $TIMESTAMP promql/impossible
+```
+
+Where `$TIMESTAMP` is either use [RFC3339](https://www.rfc-editor.org/rfc/rfc3339)
+formatted  or `YYYY-MM-DD`.
+Adding this comment will disable `promql/impossible` *until* `$TIMESTAMP`, after that
+check will be re-enabled.