man: more hyperlinks and other fixes

Closes https://github.com/systemd/systemd/issues/29814.

man/systemd.exec.xml

@@ -232,10 +232,11 @@
 
         <para>To make sure making ephemeral copies can be made efficiently, the root directory or root image
         should be located on the same filesystem as <filename>/var/lib/systemd/ephemeral-trees/</filename>.
-        When using <varname>RootEphemeral=</varname> with root directories, btrfs should be used as the
-        filesystem and the root directory should ideally be a subvolume which <command>systemd</command> can
-        snapshot to make the ephemeral copy. For root images, a filesystem with support for reflinks should
-        be used to ensure an efficient ephemeral copy.</para>
+        When using <varname>RootEphemeral=</varname> with root directories,
+        <citerefentry project='url'><refentrytitle url='https://btrfs.wiki.kernel.org/index.php/Manpage/btrfs(5)'>btrfs</refentrytitle><manvolnum>5</manvolnum></citerefentry>
+        should be used as the filesystem and the root directory should ideally be a subvolume which
+        <command>systemd</command> can snapshot to make the ephemeral copy. For root images, a filesystem
+        with support for reflinks should be used to ensure an efficient ephemeral copy.</para>
 
         <xi:include href="system-only.xml" xpointer="singular"/>
 
@@ -1917,7 +1918,7 @@ BindReadOnlyPaths=/var/lib/systemd</programlisting>
 
         <para>Note that this functionality might not be available, for example if KSM is disabled in the
         kernel, or the kernel doesn't support controlling KSM at the process level through
-        <function>prctl()</function>.</para>
+        <citerefentry><refentrytitle>prctl</refentrytitle><manvolnum>2</manvolnum></citerefentry>.</para>
 
         <xi:include href="version-info.xml" xpointer="v254"/>
         </listitem>
@@ -3180,7 +3181,7 @@ StandardInputData=V2XigLJyZSBubyBzdHJhbmdlcnMgdG8gbG92ZQpZb3Uga25vdyB0aGUgcnVsZX
         <varname>RateLimitBurst=</varname> configured in
         <citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
         Note that this only applies to log messages that are processed by the logging subsystem, i.e. by
-        <citerefentry><refentrytitle>systemd-journald.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
+        <citerefentry><refentrytitle>systemd-journald.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>.
         This means that if you connect a service's stderr directly to a file via
         <varname>StandardOutput=file:…</varname> or a similar setting, the rate limiting will not be applied
         to messages written that way (but it will be enforced for messages generated via
@@ -4147,9 +4148,9 @@ StandardInputData=V2XigLJyZSBubyBzdHJhbmdlcnMgdG8gbG92ZQpZb3Uga25vdyB0aGUgcnVsZX
           <varname>FileDescriptorStoreMax=</varname> is set to a non-zero value (see
           <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>
           for details). Applications may check this environment variable before sending file descriptors to
-          the service manager via <function>sd_pid_notify_with_fds()</function> (see
-          <citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry> for
-          details).</para>
+          the service manager via
+          <citerefentry><refentrytitle>sd_pid_notify_with_fds</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
+        </para>
 
           <xi:include href="version-info.xml" xpointer="v254"/></listitem>
         </varlistentry>