#5875 bookpart-variable.ly regtest texidoc string not rendered in collated files

[Valentin Villenave]

input/regression/bookpart-variable.ly has the following texidoc string:

A @code{\bookpart} variable can be inserted in a @code{\book}.
No segfault should occur in this case.

From what I can tell there’s no syntax oddity in that regtest; however that string is nowhere to be found in the collated files HTML page.