TextDiagnostic: print remark level diagnostics in bold too

The purpose of bolding these is to make them visually distinct from
continuations (supplemental note diagnostics). Therefore, the bolding applies
to all severities _including_ remarks -- it's not in any way an indicator of
priority. Also simplify and comment.

No tests.
DiagnosticRenderer: emit basic notes as real diagnostics

Fixes terminal column wrapping and vestigial 'note:' prefixes that would appear
when using emitBasicNote().