Adds a warning when truncating sidebar section entries

[chalin]

• The first commit is the original contribution & submission from Adds a warning when truncating menu list in sidebar-tree.html #1438 by @fekete-robert, and sorry again for the git snafu!
• Second commit contains warning-message edits and whitespace cleanup
• Double the current truncation limit value, as suggested by @fekete-robert -- sidebar_menu_truncate needs to be documented #979 (comment)

[chalin]

@fekete-robert et al. PTAL

[fekete-robert]

Thanks!

[chalin]

Thanks!

Thank you @fekete-robert! 🙏