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
@@ -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.