man: document COREDUMP_TID= and COREDUMP_THREAD_NAME=

These journal fields and the matching user.coredump.tid and
user.coredump.thread_name xattrs were added in v261 by 4c7af7d9d1
("coredump: capture crashing thread ID and name"), but never
documented.

(cherry picked from commit ff2ce8dc9c)
(cherry picked from commit c03bc382bc)

man/systemd-coredump.xml

@@ -230,6 +230,19 @@ COREDUMP_FILENAME=/var/lib/systemd/coredump/core.Web….552351.….zst
         </listitem>
       </varlistentry>
 
+      <varlistentry>
+        <term><varname>COREDUMP_TID=</varname></term>
+        <term><varname>COREDUMP_THREAD_NAME=</varname></term>
+        <listitem><para>The thread number (TID) and name
+        (<filename>/proc/<replaceable>pid</replaceable>/task/<replaceable>tid</replaceable>/comm</filename>)
+        of the thread that triggered the core dump. For single-threaded processes the TID is the same as
+        the PID. Like <varname>COREDUMP_PID=</varname>, the TID is the value as seen in the namespace where
+        <filename>systemd-coredump</filename> is running.</para>
+
+        <xi:include href="version-info.xml" xpointer="v261"/>
+        </listitem>
+      </varlistentry>
+
       <varlistentry>
         <term><varname>COREDUMP_BY_PIDFD=</varname></term>
         <listitem><para>If the crashed process was analyzed using a PIDFD provided by the kernel (requires
@@ -539,6 +552,16 @@ flags:   ...
 
         <xi:include href="version-info.xml" xpointer="v248"/></listitem>
       </varlistentry>
+
+      <varlistentry>
+        <term><varname>user.coredump.tid</varname></term>
+        <term><varname>user.coredump.thread_name</varname></term>
+
+        <listitem><para>Those are the same as <varname>COREDUMP_TID=</varname> and
+        <varname>COREDUMP_THREAD_NAME=</varname>, described above.</para>
+
+        <xi:include href="version-info.xml" xpointer="v261"/></listitem>
+      </varlistentry>
     </variablelist>
 
     <para>Those can be viewed using