cd2nroff: use an empty "##" to signal end of .IP sequence

cd2nroff: use an empty "##" to signal end of .IP sequence

Like when we list a series of options and then want to add "normal" text
again afterwards.

Without this, the indentation level wrongly continues even after the
final "##" header, making following text wrongly appear to belong to the
header above.

Adjusted several curldown files to use this.

Fixes #13803
Reported-by: Jay Satiro
Closes #13806