Revert "Issue #2570431 by stefan.r, YesCT, David_Rothstein, xjm, catch,...

Revert "Issue #2570431 by stefan.r, YesCT, David_Rothstein, xjm, catch, effulgentsia, Heine, dawehner, plach, lauriii, alexpott, joelpittet: Document that certain (non-"href") attribute values in t() and SafeMarkup::format() are not supported and may be insecure"

This reverts commit 4d8e0102.