document error message terms

[jesse99]

Error messages and the documentation needs to be kept in sync. In particular terms used by error messages (e.g. "outer comment") need to be documented.

[graydon]

re-nominating for reconsideration. this is either too vague (and should close), or else we need a mechanism to check it.

possible mechanism: a docterm!(str) macro that does nothing but can be mechanically extracted and compared to docs. Other possibilities include mechanisms associated with error message translation / message catalogues. It might make sense for rustdoc to support definition-extraction in general.

see also #2092 , where we might want a way to link external doc fragments (as in a FAQ) to error messages.

[graydon]

accepted for production-ready milestone

[graydon]

language being well-defined is #1, diagnostics being well-defined is #5

[thestinger]

This is a good candidate for being a metabug, but more specific issues need to be reported.

[pnkfelix]

closing as non-actionable.