Add comment to elaborate

src/bootstrap/bootstrap.py

@@ -313,6 +313,12 @@ def default_build_triple(verbose):
         # these cases, fall back to using sys.platform.
         return 'x86_64-pc-windows-msvc'
     elif kernel == 'AIX':
+        # `uname -m` returns the machine ID rather than machine hardware on AIX,
+        # so we are unable to use cputype to form triple. Since AIX 7.2 and
+        # above supports 32-bit and 64-bit mode simultaneously and `uname -p`
+        # returns `powerpc`. Currently we only supports `powerpc64-ibm-aix` in
+        # rust. For above reasons, kerneltype_mapper and cputype_mapper are not
+        # used to infer AIX's triple.
         return 'powerpc64-ibm-aix'
     else:
         err = "unknown OS type: {}".format(kernel)