Document limitation of BusyBox tr

[Bruno Haible]

Investigating a test failure of GNU grep on Alpine Linux 3.17
<https://lists.gnu.org/archive/html/grep-devel/2023-03/msg00003.html>
I found that the cause is:

The 'tr' program on Alpine Linux (origin: BusyBox 1.35.0) does not implement
the [x*n] argument syntax that POSIX mandates
<https://pubs.opengroup.org/onlinepubs/9699919799/utilities/tr.html>

Here's a proposed patch to the Autoconf documentation, chapter "Limitations
of Usual Tools", to document this. We already documentation limitations in
BusyBox 'sed' and BusyBox 'join' in the same chapter.

0001-Document-limitation-of-BusyBox-tr.patch
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