[docs] Document coding standard for error and warning messages

[docs] Document coding standard for error and warning messages

In particular, these messages should start with a lower-case letter and
should have no trailing period at the end of the last sentence.

See http://lists.llvm.org/pipermail/llvm-dev/2020-March/140178.html for
context.

Reviewed by: aaron.ballman, hubert.reinterpretcast, rnk, dblaikie

Differential Revision: https://reviews.llvm.org/D76833