Add documentation to PhysicsServer2DExtension #321

Job	Run time
📊 Static checks / Code style, file formatting, and docs	2m 36s
🍏 iOS / Template (target=template_release)	16m 59s
🤖 Android / Template (target=template_release)	27m 41s
🍎 macOS / Editor (target=editor, tests=yes)	26m 59s
🍎 macOS / Template (target=template_release)	19m 13s
🌐 Web / Template (target=template_release)	20m 39s
🏁 Windows / Editor (target=editor, tests=yes)	41m 46s
🏁 Windows / Template (target=template_release)	29m 29s
🐧 Linux / Editor w/ Mono (target=editor)	35m 13s
🐧 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)	36m 36s
🐧 Linux / Editor with clang sanitizers (target=editor, tests=yes, dev_build=yes, use_asan=yes, use_ubsan=yes, use_llvm=yes, linker=lld)	31m 56s
🐧 Linux / Editor with ThreadSanitizer (target=editor, tests=yes, dev_build=yes, use_tsan=yes, use_llvm=yes, linker=lld)	27m 26s
🐧 Linux / Template w/ Mono (target=template_release)	22m 50s
🐧 Linux / Minimal template (target=template_release, everything disabled)	13m 51s
🪲 Godot CPP / Build and test Godot CPP	6m 42s
	5h 59m 56s

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