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 +162,10 @@ 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. The minimum versions listed here
+    correspond to the binaries provided by the
+    [unofficial-builds project](https://github.com/nodejs/unofficial-builds).
+
 <!--lint enable final-definition-->
 
 ### Supported toolchains