more documentation for TMPDIR

mmdebstrap

@@ -7032,7 +7032,12 @@ or fill them with reproducible content:
 When creating a tarball, a temporary directory is populated with the rootfs
 before the tarball is packed. The location of that temporary directory will be
 in F</tmp> or the location pointed to by C<TMPDIR> if that environment variable
-is set.
+is set. Setting C<TMPDIR> to a different directory than F</tmp> is useful if
+you have F</tmp> on a tmpfs that is too small for your rootfs.
+
+If you set C<TMPDIR> in B<unshare> mode, then the unshared user must be able to
+access the directory. This means that the directory itself must be
+world-writable and all its ancestors must be at least world-executable.
 
 Since C<TMPDIR> is only valid outside the chroot, the variable is being unset
 when running hook scripts. If you need a valid temporary directory in a hook,