Nixos manual: nixos-rebuild boot or switch to clear boot entries

the `nixos-rebuild` command has multiple subcommands, and not each of
them would fix the problem of a large `/boot` partition, so let’s be
more precise here.

Changed files
+4 -3
nixos
doc
manual
administration
from_md
+2 -2
nixos/doc/manual/administration/cleaning-store.chapter.md
···
## NixOS Boot Entries {#sect-nixos-gc-boot-entries}
If your `/boot` partition runs out of space, after clearing old profiles
-
you must rebuild your system with `nixos-rebuild` to update the `/boot`
-
partition and clear space.
+
you must rebuild your system with `nixos-rebuild boot` or `nixos-rebuild
+
switch` to update the `/boot` partition and clear space.
+2 -1
nixos/doc/manual/from_md/administration/cleaning-store.chapter.xml
···
<para>
If your <literal>/boot</literal> partition runs out of space,
after clearing old profiles you must rebuild your system with
-
<literal>nixos-rebuild</literal> to update the
+
<literal>nixos-rebuild boot</literal> or
+
<literal>nixos-rebuild switch</literal> to update the
<literal>/boot</literal> partition and clear space.
</para>
</section>