doc: clarify version column in experimental tier

Author: kleisauke

BUILDING.md

@@ -128,7 +128,7 @@ releases for these platforms. Test failures on experimental platforms do not
 block releases. Contributions to improve support for these platforms are
 welcome.
 
-| Operating System | Architectures    | Versions                      | Notes                       |
+| Operating System | Architectures    | Versions[^5]                  | Notes                       |
 | ---------------- | ---------------- | ----------------------------- | --------------------------- |
 | GNU/Linux        | x64              | kernel >= 6.6, musl >= 1.2.4  | e.g. Alpine 3.19            |
 | GNU/Linux        | x86              | kernel >= 3.10, glibc >= 2.17 | Downgraded as of Node.js 10 |
@@ -162,6 +162,8 @@ welcome.
 [^4]: Our macOS Binaries are compiled with 13.5 as a target. Xcode 16 is
     required to compile.
 
+[^5]: Older kernel or libc versions may work.
+
 <!--lint enable final-definition-->
 
 ### Supported toolchains
@@ -180,13 +182,13 @@ Binaries at <https://nodejs.org/download/release/> are produced on:
 
 | Binary package          | Platform and Toolchain                                        |
 | ----------------------- | ------------------------------------------------------------- |
-| aix-ppc64               | AIX 7.2 TL04 on PPC64BE with GCC 12[^5]                       |
+| aix-ppc64               | AIX 7.2 TL04 on PPC64BE with GCC 12[^6]                       |
 | darwin-x64              | macOS 15, Xcode 16 with -mmacosx-version-min=13.5             |
 | darwin-arm64 (and .pkg) | macOS 15 (arm64), Xcode 16 with -mmacosx-version-min=13.5     |
-| linux-arm64             | RHEL 8 with Clang 19.1 and gcc-toolset-14-libatomic-devel[^6] |
-| linux-ppc64le           | RHEL 8 with Clang 19.1 and gcc-toolset-14-libatomic-devel[^6] |
-| linux-s390x             | RHEL 8 with Clang 19.1 and gcc-toolset-14-libatomic-devel[^6] |
-| linux-x64               | RHEL 8 with Clang 19.1 and gcc-toolset-14-libatomic-devel[^6] |
+| linux-arm64             | RHEL 8 with Clang 19.1 and gcc-toolset-14-libatomic-devel[^7] |
+| linux-ppc64le           | RHEL 8 with Clang 19.1 and gcc-toolset-14-libatomic-devel[^7] |
+| linux-s390x             | RHEL 8 with Clang 19.1 and gcc-toolset-14-libatomic-devel[^7] |
+| linux-x64               | RHEL 8 with Clang 19.1 and gcc-toolset-14-libatomic-devel[^7] |
 | win-arm64               | Windows Server 2022 (x64) with Visual Studio 2022             |
 | win-x64                 | Windows Server 2022 (x64) with Visual Studio 2022             |
 
@@ -197,10 +199,10 @@ on your Linux distribution.
 
 <!--lint disable final-definition-->
 
-[^5]: Binaries produced on these systems require libstdc++12, available
+[^6]: Binaries produced on these systems require libstdc++12, available
     from the [AIX toolbox][].
 
-[^6]: Binaries produced on these systems are compatible with glibc >= 2.28
+[^7]: Binaries produced on these systems are compatible with glibc >= 2.28
     and libstdc++ >= 6.0.25 (`GLIBCXX_3.4.25`). These are available on
     distributions natively supporting GCC 8.1 or higher, such as Debian 10,
     RHEL 8 and Ubuntu 20.04.