Document flag:0 for no flags in page Searching (#440)

* Document flag:0 for no flags in page Searching

* Wording in flags section of searching.md

edited to maintain singular

---------

Co-authored-by: Danika-Dakika <115673540+Danika-Dakika@users.noreply.github.com>

Author: benjaminhottell

src/searching.md

@@ -263,6 +263,9 @@ cards that are in learning for the first time.
 
 ## Flags
 
+`flag:0`\
+cards without a flag.
+
 `flag:1`\
 cards with a red flag.