[DOCS] Clarify regex character range case insensitivity limitations

Flagged by @dadoonet in Slack:

> Hey team. There's an interesting discussion on discuss about Lucene regex support.
Our documentation says things like:
[a-c]   # matches 'a', 'b', or 'c'
[^a-c]  # matches any character except 'a', 'b', or 'c'
Which is correct unless you use as well case_insensitive: true option. In which case you would expect B to match [a-c]. But it does not work that way and it's a known limitation as Robert answered in apache/lucene#14378 (comment).
Could we document that somehow?

Author: leemthompo

docs/reference/query-dsl/regexp-syntax.asciidoc

@@ -181,6 +181,15 @@ example:
 [^-abc]     # matches any character except '-', 'a', 'b', or 'c'
 [^abc\-]    # matches any character except 'a', 'b', 'c', or '-'
 ....
+
+[NOTE]
+====
+Character range classes such as `[a-c]` do not behave as expected when using `case_insensitive: true` — they remain case sensitive.
+For example, `[a-c]+` with `case_insensitive: true` will match strings containing only the characters 'a', 'b', and 'c', but not 'A', 'B', or 'C'. 
+Use `[a-zA-Z]` to match both uppercase and lowercase characters.
+This is due to a known limitation in Lucene's regular expression engine.
+See https://github.com/apache/lucene/issues/14378[Lucene issue #14378] for details.
+====
 --
 
 [discrete]