[Doc] Clarify `is_null` methods of `Callable` and `Signal` #37470

Job	Run time
📊 Static checks / Code style, file formatting, and docs	30s
🍏 iOS / Template (target=template_release)	1m 26s
🍎 macOS / Editor (target=editor, tests=yes)	3m 25s
🌐 Web / Template w/ threads (target=template_release, threads=yes)	6m 3s
🐧 Linux / Editor w/ Mono (target=editor)	4m 45s
🍎 macOS / Template (target=template_release)	3m 2s
🌐 Web / Template w/o threads (target=template_release, threads=no)	6m 51s
🏁 Windows / Editor (target=editor, tests=yes)	8m 6s
🐧 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)	10m 25s
🏁 Windows / Template (target=template_release)	7m 57s
🐧 Linux / Editor with clang sanitizers (target=editor, tests=yes, dev_build=yes, use_asan=yes, use_ubsan=yes, use_llvm=yes, linker=lld)	9m 25s
🐧 Linux / Editor with ThreadSanitizer (target=editor, tests=yes, dev_build=yes, use_tsan=yes, use_llvm=yes, linker=lld)	6m 13s
🐧 Linux / Template w/ Mono (target=template_release)	2m 14s
🤖 Android / Editor (target=editor)	3m 56s
🐧 Linux / Minimal template (target=template_release, everything disabled)	3m 41s
🤖 Android / Template arm32 (target=template_release, arch=arm32)	3m 1s
🤖 Android / Template arm64 (target=template_release, arch=arm64)	3m 0s
🪲 Godot CPP / Build and test Godot CPP	8m 25s
	1h 32m 25s

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