[support.srcloc.class] Highlight unspecified properties (#3309)

of the source_location constructors and copy assignment
operators.

Author: jensmaurer

source/support.tex

@@ -3185,6 +3185,9 @@
 \begin{note}
 The intent of \tcode{source_location} is
 to have a small size and efficient copying.
+It is unspecified
+whether the copy/move constructors and the copy/move assignment operators
+are trivial and/or constexpr.
 \end{note}
 
 \pnum