docs: call out breaking changes as experimental

Explicitly calls out that recent breaking changes were only for
experimental features

Author: mmalerba

CHANGELOG.md

@@ -47,7 +47,7 @@
 
 <a name="21.0.6"></a>
 # 21.0.6 (2025-12-17)
-## Breaking Changes
+## Breaking Changes (affecting only experimental features)
 ### forms
 - The shape of `SignalFormsConfig.classes` has changed
 
@@ -8353,4 +8353,4 @@ Adrien Crivelli, Alan Agius, Alex Rickabaugh, Andrew Kushnir, Andrew Scott, Dyla
 
 Adrian Kunz, Alan Agius, AleksanderBodurri, Alex Rickabaugh, AlirezaEbrahimkhani, Amir Rustamzadeh, Andrew Kushnir, Andrew Scott, Chabbey François, Charles Lyding, Cédric Exbrayat, Daan De Smedt, David Schmidt, Derek Cormier, Dmitrij Kuba, Doug Parker, Dylan Hunn, Emma Twersky, George Kalpakas, George Looshch, Jan Kuehle, Jessica Janiuk, JiaLiPassion, JimMorrison723, Joe Martin (Crowdstaffing), Joey Perrott, JoostK, Kristiyan Kostadinov, Krzysztof Platis, Leosvel Pérez Espinosa, Maddie Klein, Mark Whitfeld, Martin Sikora, Michael-Doner, Michal Materowski, Minko Gechev, Paul Gschwendtner, Pawel Kozlowski, Payam Shahidi, Pusztai Tibor, Ricardo Mattiazzi Baumgartner, Roy Dorombozi, Ruslan Lekhman, Samuel Littley, Sergej Grilborzer, Sumit Arora, Tobias Speicher, Virginia Dooley, Zack Elliott, alirezaghey, ananyahs96, arturovt, cexbrayat, dario-piotrowicz, ivanwonder, kamikopi, markostanimirovic, markwhitfeld, mgechev, renovate[bot], twerske and zverbeta
 
-Earlier changelog entries can be found in CHANGELOG_ARCHIVE.md file.
+Earlier changelog entries can be found in CHANGELOG_ARCHIVE.md file.