docs/cross_compiling: fix makedeps host/build desc

* makedepends_build is for the native chroot
* makedepends_host is for the foreign chroot

I had it the other way around in the documentation. The variables are so
far not documented here yet:
* https://gitlab.alpinelinux.org/alpine/abuild/-/blob/master/APKBUILD.5.scd
* https://wiki.alpinelinux.org/wiki/APKBUILD_Reference

But it now matches the comments Caleb had written for the cross-native2
implementation, and also what can be found in e.g. the abuild APKBUILD
in aports.git:

  makedepends_build="pkgconfig scdoc"
  makedepends_host="openssl-dev>3 zlib-dev"

Part-of: !2566
[ci:skip-build]: already built successfully in CI

docs/cross_compiling.md

@@ -68,11 +68,11 @@ Enable this method with `options="pmb:cross-native2"` in the `APKBUILD`.
 This is supported since pmbootstrap 3.4, previous versions will ignore this
 option and use the crossdirect method instead.
 
-Packages from `makedepends_host=` in the `APKBUILD` are installed in the native
-chroot, and packages from `makedepends_build=` in the foreign chroot.
-Environment variables are used to run abuild in the native chroot and use
-its cross compilation features rather than running it and the build
-system through QEMU. This massively speeds up building when it works.
+Packages from `makedepends_build=` in the `APKBUILD` are installed in the
+native chroot (where we run the build), and packages from `makedepends_host=`
+in the foreign chroot. Environment variables are used to run abuild in the
+native chroot and use its cross compilation features rather than running it and
+the build system through QEMU. This massively speeds up building when it works.
 
 The foreign chroot is mounted into the native chroot at `/sysroot`. Most build
 systems will refer to it as "sysroot", in abuild it is `CBUILDROOT`.