main/busybox: fix empty -doc

Resolves #11443

---
The man page in the busybox-doc package seems to be empty.

    $ apk info -L busybox-doc
    busybox-doc-1.31.1-r14 contains:
    usr/share/man/man1/busybox.1.gz

    $ zcat /usr/share/man/man1/busybox.1.gz
    $
---

/bin/sh: pod2text: not found
  DOC     busybox.1
make[2]: [/home/buildozer/aports/main/busybox/src/busybox-1.31.1/Makefile.custom:159: docs/BusyBox.txt] Error 127 (ignored)
/bin/sh: pod2man: not found
make[2]: [/home/buildozer/aports/main/busybox/src/busybox-1.31.1/Makefile.custom:164: docs/busybox.1] Error 127 (ignored)
/bin/sh: pod2html: not found
make[2]: [/home/buildozer/aports/main/busybox/src/busybox-1.31.1/Makefile.custom:174: docs/busybox.net/BusyBox.html] Error 127 (ignored)
7 jobs for !7294 with main/busybox in 5 minutes and 40 seconds
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Status Job ID Name Coverage
  Lint
failed #104801
x86_64 docker-alpine allowed to fail
lint

00:00:22

 
  Build
passed #104806
aarch64 ci-build docker-alpine
build-aarch64

00:03:32

passed #104807
armv7 ci-build docker-alpine
build-armv7

00:02:25

passed #104805
ppc64le ci-build docker-alpine
build-ppc64le

00:04:08

passed #104804
s390x ci-build docker-alpine
build-s390x

00:01:09

passed #104803
x86 ci-build docker-alpine
build-x86

00:01:04

passed #104802
x86_64 ci-build docker-alpine
build-x86_64

00:00:57

 
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