[assertions.assert] The meaning of "standard error stream" is unclear

[frederick-vs-ja]

Currently, the term is used only once in the working draft, and its definition is absent. Presumably it is defined by the C standard (see WG14 N3220 7.23.3/7). A cross reference seems helpful.

Not sure whether this can be handled editorially, or should be addressed by an LWG issue or P3348. (P3348R2 hasn't touched this yet.)

[jensmaurer]

Editorially adding a cross-reference to ISO C 7.23.3 seems fine. Pull request welcome.