release-3.1: generate_man: Prevent sibling node loss in man page generation

[slusarz]

Parent's children array was being replaced entirely with the processed nodes of a single child, causing all sibling nodes to be discarded.

Use splice to insert the processed nodes into the parent's children array in place, preserving siblings.

Example diff (doveadm-who.1):

< Global doveadm(1)

Global doveadm(1) \fIoptions\fR:
28c28
< Do not read any config file, just use defaults. The \fBdovecot_storage_version\fR setting defaults to the latest version, but can be overridden with ---
Do not read any config file, just use defaults. The \fBdovecot_storage_version\fR setting defaults to the latest version, but can be overridden with \fB-o\fR.
150c150
< By default doveadm(1) will use the socket

---

By default doveadm(1) will use the socket \fI/rundir/anvil\fR. The socket may be located in another directory, when the default \fIbase_dir\fR setting was overridden in \fI/etc/dovecot/dovecot.conf\fR.