docs: describe the search text limits in schema.graphql

backend/src/schema.graphql

@@ -198,7 +198,10 @@ type Query {
     One or more words, case-insensitive. All the words must match: an item is
     found if, for every search word, it has a word that starts with it, in the
     code, name or description. A trailing "s" is removed from each search
-    word. Empty or missing means no text filter.
+    word. At most 80 characters and 8 words, otherwise BAD_USER_INPUT. Text
+    that has no searchable words (such as "ø" or "%") finds nothing. Empty or
+    only spaces means no text filter. A client should limit its input field to
+    80 characters.
     """
     search: String
     filter: CatalogFilter