Fixes T8845. Ref T13102. See PHI467. Currently, object monograms like L1 which appear in Remarkup headers render incorrectly (with an internal placeholder "x") in the table of contents:
Instead, render them down to just, e.g., L1 in plain text.
For {P123} I just rendered it to {P123} since it's not really clear to me what users intend. This could be adjusted if there's some reasonable thing that someone is trying to do with this.