Merge pull request #9775 from yuwata/follow-up-9766

diff --cc man/systemd.network.xml
index 8a80b0cf4fe4c2742ca3bb906c64a9615e04cc1d,c3dd8d07a917bac0b62c9ab1ca60d4cc22fb5b80..cbf53f323abac9e5525596a07afeb1531e8d9195
--- 1/man/systemd.network.xml
--- 2/man/systemd.network.xml
+++ b/man/systemd.network.xml
@@@ -1194,8 -1194,10 +1194,10 @@@
          <varlistentry>
            <term><varname>SendHostname=</varname></term>
            <listitem>
-             <para>When true (the default), the machine's hostname will
-             be sent to the DHCP server.</para>
+             <para>When true (the default), the machine's hostname will be sent to the DHCP server.
+             Note that the machine's hostname must consist only of 7-bit ASCII lower-case characters and
+             no spaces or dots, and be formatted as a valid DNS domain name. Otherwise, the hostname is not
 -            send even if this is set to true.</para>
++            sent even if this is set to true.</para>
            </listitem>
          </varlistentry>
          <varlistentry>