"Register-true" labelling has changed, see bug 127279. Relevant help pages need updating.
Let's put ASSIGNED since you assigned yourself to fix this.
Seth Chaiklin committed a patch related to this issue. It has been pushed to "master": https://git.libreoffice.org/help/commit/62b3cf8ef046ea0ad790dadb8735a9a6662282ea tdf#137221 Update help page with Register-true
Seth Chaiklin committed a patch related to this issue. It has been pushed to "master": https://git.libreoffice.org/help/commit/218e253ba66a9459c4bbd92b59bee1c695fb0244 tdf#137221 add Register-true to Glossary as separate entry
Seth Chaiklin committed a patch related to this issue. It has been pushed to "master": https://git.libreoffice.org/help/commit/f9f9dd593ca29d23fec10381f3fd70e3bebfd8d1 tdf#137221 make corrections in help page for using "Page line-spacing"
Seth Chaiklin committed a patch related to this issue. It has been pushed to "master": https://git.libreoffice.org/help/commit/d13a2e451488a17904e13a4060ccca8741aa7763 tdf#137221 change link in "register-true" in Glossary
Seth Chaiklin committed a patch related to this issue. It has been pushed to "master": https://git.libreoffice.org/help/commit/72f29429460b16b0b15838fe70627995e7f8cead tdf#137221 change link for Page line-spacing