Add missing documentation for `Skeleton3D` methods #15420

Job	Run time
📊 Static checks / Code style, file formatting, and docs	36s
🤖 Android / Template (target=template_release)	2m 53s
🍏 iOS / Template (target=template_release)	2m 35s
🌐 Web / Template (target=template_release)	1m 7s
🍎 macOS / Editor (target=editor, tests=yes)	4m 39s
🍎 macOS / Template (target=template_release)	5m 43s
🏁 Windows / Editor (target=editor, tests=yes)	7m 48s
🐧 Linux / Editor w/ Mono (target=editor)	4m 37s
🏁 Windows / Template (target=template_release)	10m 3s
🐧 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)	17m 55s
🐧 Linux / Editor with clang sanitizers (target=editor, tests=yes, dev_build=yes, use_asan=yes, use_ubsan=yes, use_llvm=yes, linker=lld)	8m 50s
🐧 Linux / Editor with ThreadSanitizer (target=editor, tests=yes, dev_build=yes, use_tsan=yes, use_llvm=yes, linker=lld)	5m 39s
🐧 Linux / Template w/ Mono (target=template_release)	1m 36s
🐧 Linux / Minimal template (target=template_release, everything disabled)	1m 40s
🪲 Godot CPP / Build and test Godot CPP	10m 35s
	1h 26m 16s

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