mandoc cannot handle XZ compression (e.g. sysstat-doc) in 3.15

Hi,

I upgraded to 3.15 yesterday on one of my home Linux boxes and today wanted to monitor disk I/O, so I installed sysstat and systat-doc. The surprise came at "man iostat":

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ý7zXZ???æÖ´F??!?????t/å£àBî?Ö]????B???ççÇÄ�#sSÌ¥·ú×,þð�?çÃ?1õ;HíÂs�¬�Ìüï�Bl,B�.�¢��àFµé¾��s×[õ_Qz�IË~ÿ 9�ÓÈô�iª8ýD
y]òLC^{_?�õZLŽ??I)a?�F?Ï*�S�ÓR_�º°Y?ÌÛ?¨â?��oõU[NâqàÃ%?¾���Y�?Ãñt    ??D?¹�W¢
?Ìg­>�~.¾¬púÈ?Yú.¡±ý?ÄP?@Ä�¶�ÙÎøþå_3ÂX¢�± [|> ç}�J,?�r8ZÜu½?I¨

etc. etc.

Less says the file is /tmp/man.XXXXbLEKJn, so let's see:

/tmp/man.XXXXbLEKJn: Unicode text, UTF-8 text, with very long lines (334)

It's obviously a red herring. A quick check shows that man pages in sysstat-doc are all compressed with xz instead of gzip, and I assume that mandoc cannot yet handle this compressor.

# apk info -L sysstat-doc
sysstat-doc-12.5.4-r0 contains:
usr/share/doc/sysstat-12.5.4/CHANGES
usr/share/doc/sysstat-12.5.4/COPYING
usr/share/doc/sysstat-12.5.4/CREDITS
usr/share/doc/sysstat-12.5.4/FAQ.md
usr/share/doc/sysstat-12.5.4/README.md
usr/share/doc/sysstat-12.5.4/sysstat-12.5.4.lsm
usr/share/man/man1/cifsiostat.1.xz
usr/share/man/man1/iostat.1.xz
usr/share/man/man1/mpstat.1.xz
usr/share/man/man1/pidstat.1.xz
usr/share/man/man1/sadf.1.xz
usr/share/man/man1/sar.1.xz
usr/share/man/man1/tapestat.1.xz
usr/share/man/man5/sysstat.5.xz
usr/share/man/man8/sa1.8.xz
usr/share/man/man8/sa2.8.xz
usr/share/man/man8/sadc.8.xz