add note about regex (#3437)

to prevent people from running into https://github.com/grafana/loki/issues/3436

docs/sources/logql/_index.md

@@ -74,6 +74,8 @@ Examples:
 
 The same rules that apply for [Prometheus Label Selectors](https://prometheus.io/docs/prometheus/latest/querying/basics/#instant-vector-selectors) apply for Loki log stream selectors.
 
+**Important note:** The `=~` regex operator is fully anchored, meaning regex must match against the *entire* string, including newlines. The regex `.` character does not match newlines by default. If you want the regex dot character to match newlines you can use the single-line flag, like so: `(?s)search_term.+` matches `search_term\n`.
+
 ### Log Pipeline
 
 A log pipeline can be appended to a log stream selector to further process and filter log streams. It usually is composed of one or multiple expressions, each expressions is executed in sequence for each log line. If an expression filters out a log line, the pipeline will stop at this point and start processing the next line.