SL.io.50 (Avoid endl): Mention string streams

Explicitly mentioned string streams as `endl` insertions into string streams do actually occur in the wild.

Author: N-Dekker

CppCoreGuidelines.md

@@ -20308,6 +20308,11 @@ For writing to a file, there is rarely a need to `flush`.
 
 ##### Note
 
+For string streams (specifically `ostringstream`), the insertion of an `endl` is entirely equivalent
+to the insertion of a `'\n'` character, but also in this case, `endl` might be slightly slower.
+
+##### Note
+
 Apart from the (occasionally important) issue of performance,
 the choice between `'\n'` and `endl` is almost completely aesthetic.