AK: Document the non-standard extensions in TRY

I'm not sure there's a material improvement from this patch. However,
I've been reading the error handling code from multiple projects and
was excited to see Serenity being able to handle assignment
(`auto x = TRY(make_x())`) the same way as actions (`TRY(do_x())`).
I think it's worth documenting that this is only possible due to
non-standard extensions.

AK/Try.h

@@ -8,6 +8,11 @@
 
 // NOTE: This macro works with any result type that has the expected APIs.
 // It's designed with AK::Result and AK::Error in mind.
+//
+// It depends on a non-standard C++ extension, specifically
+// on statement expressions [1]. This is known to be implemented
+// by at least clang and gcc.
+// [1] https://gcc.gnu.org/onlinedocs/gcc/Statement-Exprs.html
 
 #define TRY(expression) \
  ({ \