resolved: document that IPC provided and networkd provided DNS servers are combined, and the former cannot override the latter

systemd version the issue has been seen with
systemd 245 (245.5-2-arch)
+PAM +AUDIT -SELINUX -IMA -APPARMOR +SMACK -SYSVINIT +UTMP +LIBCRYPTSETUP +GCRYPT +GNUTLS +ACL +XZ +LZ4 +SECCOMP +BLKID +ELFUTILS +KMOD +IDN2 -IDN +PCRE2 default-hierarchy=hybrid

Used distribution
Arch Linux

Unexpected behaviour you saw
I expect that resolvectl dns LINK ADDR sets the nameservers for LINK to ADDR. However, it appends ADDR to the existing list.

Attempting to remove all the addresses using resolvectl LINK '' results in the dns servers being reverted to the DHCP assigned nameservers.

I would expect the nameservers to be replaced fully with the ADDR (or provide a switch that allows me to do so).

Steps to reproduce the problem

1. Have an interface that is dhcp configured, but you want to use a different nameserver at runtime
2. call resolvectl dns LINK ADDR
3. The server is added to the list, the list is not replaced

Start with the documentation for the resolvectl dns LINK ADDR command and the description of DHCP-provided DNS servers. Document that IPC-provided and networkd-provided servers are combined, and that the former cannot override the latter; confirm the wording against the reproduction steps and expected behavior in this issue.