Add documentation about filters

Signed-off-by: Olga Bulat <[email protected]>

documentation/api/reference/search_algorithm.md

@@ -136,11 +136,12 @@ following fields:
 
 - Extension
 - Category
-- Length
-- Aspect ratio
-- Size
 - Source
 - License
+- License type
+- Length (audio only)
+- Aspect ratio (image only)
+- Size (image only)
 
 Source is the only field for which you can currently also specify exclusions.
 
@@ -152,12 +153,13 @@ field:
 - [Audio search](https://api.openverse.engineering/v1/#operation/audio_search)
 - [Image search](https://api.openverse.engineering/v1/#operation/image_search)
 
-Each of these fields are searched relatively strictly, primarily because the
-search domain in each is very small and "keyword" like. That is, there is a
-limited and specific set of terms that appear for the relevant document fields
-for each of these query parameters. All of them are validated to only allow
-specific options (documented in the API documentation links above), which
-enforces the "keyword" like nature of their usage.
+For each of these fields, there is a limited and specific set of terms that
+appear for the relevant document fields for each of these query parameters.
+These fields are matched exactly, using the filter context Elasticsearch queries
+("filter" or "must_not"). Filter-context queries can be cached by Elasticsearch,
+which improves their performance. All of these filters except for `extension`
+are validated to only allow specific options (documented in the API
+documentation links above).
 
 ### General "query" searching