add instructor notes about broadcast messages

[brownsarahm]

adds ntoes to the two live coding practices about when to send broadcast messages during the teaching.

closes #1947

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🔍 Inspect the changes: https://github.com/carpentries/instructor-training/compare/md-outputs..md-outputs-PR-2016

The following changes were observed in the rendered markdown documents:

 17-live.md        | 11 +++++++++++
 20-performance.md | 13 +++++++++++++
 md5sum.txt        |  4 ++--
 3 files changed, 26 insertions(+), 2 deletions(-)

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⏱️ Updated at 2026-06-04 17:53:47 +0000