Update string_search_functions.md

docs/en/query_language/functions/string_search_functions.md

 # Functions for Searching Strings
 
-The search is case-sensitive in all these functions.
+The search is case-sensitive by default in all these functions. There are separate variants for case insensitive search.
 The search substring or regular expression must be a constant in all these functions.
 
 ## position(haystack, needle), locate(haystack, needle)