Add missing descriptions to `PhysicalBone3D` class. #41505

Job	Run time
📊 Static checks / Code style, file formatting, and docs	31s
🤖 Android / Editor (target=editor)	5m 11s
🤖 Android / Template arm32 (target=template_release, arch=arm32)	3m 20s
🍏 iOS / Template (target=template_release)	2m 25s
🐧 Linux / Editor w/ Mono (target=editor)	3m 33s
🤖 Android / Template arm64 (target=template_release, arch=arm64)	3m 23s
🐧 Linux / Editor with doubles and GCC sanitizers (target=editor, tests=yes, dev_build=yes, scu_build=yes, precision=double, use_asan=yes, use_ubsan=yes, linker=gold)	11m 46s
🐧 Linux / Editor with clang sanitizers (target=editor, tests=yes, dev_build=yes, use_asan=yes, use_ubsan=yes, use_llvm=yes, linker=lld)	7m 31s
🐧 Linux / Editor with ThreadSanitizer (target=editor, tests=yes, dev_build=yes, use_tsan=yes, use_llvm=yes, linker=lld)	7m 37s
🐧 Linux / Template w/ Mono (target=template_release, tests=yes)	1m 8s
🍎 macOS / Editor (target=editor, tests=yes)	4m 1s
🌐 Web / Template w/ threads (target=template_release, threads=yes)	6m 4s
🐧 Linux / Minimal template (target=template_release, tests=yes, everything disabled)	59s
🌐 Web / Template w/o threads (target=template_release, threads=no)	7m 15s
🍎 macOS / Template (target=template_release, tests=yes)	20m 21s
🏁 Windows / Editor (target=editor, tests=yes)	8m 56s
🏁 Windows / Editor w/ clang-cl (target=editor, tests=yes, use_llvm=yes)	4m 25s
🏁 Windows / Template (target=template_release, tests=yes)	8m 10s
🪲 Godot CPP / Build and test Godot CPP	8m 34s
	1h 55m 10s

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