Document the safety properties of lazy"str"

base/strings/lazy.jl

@@ -47,6 +47,15 @@ LazyString
 
 !!! compat "Julia 1.8"
     `lazy"str"` requires Julia 1.8 or later.
+
+# Extended help
+## Safety properties for concurrent programs
+
+A lazy string itself does not introduce any concurrency problems even if it is printed in
+multiple Julia tasks.  However, if `print` methods on a captured value can have a
+concurrency issue when invoked without synchronizations, printing the lazy string may cause
+an issue.  Furthermore, the `print` methods on the captured values may be invoked multiple
+times.
 """
 macro lazy_str(text)
     parts = Any[]