demo validate and __call__

Co-authored-by: zsimjee <[email protected]>

docs/concepts/validators.md

@@ -12,7 +12,7 @@ Each validator is a method that encodes some criteria, and checks if a given val
 
 Occasionally, validators need additional metadata that is only available during runtime. Metadata could be data generated during the execution of a validator (*important if you're writing your own validators*), or could just be a container for runtime arguments.
 
-As an example, the `ExtractedSummarySentencesMatch` validator accepts a `filepaths` property in the metadata dictionary to specify what source files to compare the summary against to ensure similarity.  Unlike arguments which are specified at validator initialization, metadata is specified when calling `guard.validate`. For more information on how to use metadata, see [How to use Metadata](/docs/hub/how_to_guides/metadata.md).
+As an example, the `ExtractedSummarySentencesMatch` validator accepts a `filepaths` property in the metadata dictionary to specify what source files to compare the summary against to ensure similarity.  Unlike arguments which are specified at validator initialization, metadata is specified when calling `guard.validate` or `guard.__call__` (this is the `guard()` function). For more information on how to use metadata, see [How to use Metadata](/docs/hub/how_to_guides/metadata.md).
 
 ```python
 guard = Guard.from_rail("my_railspec.rail")