man: document automatic dependencies

For all units ensure there's an "Automatic Dependencies" section in the
man page, and explain which dependencies are automatically added in all
cases, and which ones are added on top if DefaultDependencies=yes is
set.

This is also done for systemd.exec(5), systemd.resource-control(5) and
systemd.unit(5) as these pages describe common behaviour of various unit
types.

man/systemd.exec.xml

@@ -76,6 +76,31 @@
     unit type.</para>
   </refsect1>
 
+  <refsect1>
+    <title>Automatic Dependencies</title>
+
+    <para>A few execution parameters result in additional, automatic
+    dependencies to be added.</para>
+
+    <para>Units with <varname>WorkingDirectory=</varname> or
+    <varname>RootDirectory=</varname> set automatically gain
+    dependencies of type <varname>Requires=</varname> and
+    <varname>After=</varname> on all mount units required to access
+    the specified paths. This is equivalent to having them listed
+    explicitly in <varname>RequiresMountsFor=</varname>.</para>
+
+    <para>Similar, units with <varname>PrivateTmp=</varname> enabled
+    automatically get mount unit dependencies for all mounts
+    required to access <filename>/tmp</filename> and
+    <filename>/var/tmp</filename>.</para>
+
+    <para>Units whose output standard output or error output is
+    connected to any other sink but <option>null</option>,
+    <option>tty</option> and <option>socket</option> automatically
+    acquire dependencies of type <varname>After=</varname> on
+    <filename>journald.socket</filename>.</para>
+  </refsect1>
+
   <refsect1>
     <title>Options</title>
 
@@ -93,7 +118,9 @@
         and the respective user's home directory if run as user. If
         the setting is prefixed with the <literal>-</literal>
         character, a missing working directory is not considered
-        fatal.</para></listitem>
+        fatal. Note that setting this parameter might result in
+        additional dependencies to be added to the unit (see
+        above).</para></listitem>
       </varlistentry>
 
       <varlistentry>
@@ -104,7 +131,9 @@
         project='man-pages'><refentrytitle>chroot</refentrytitle><manvolnum>2</manvolnum></citerefentry>
         system call. If this is used, it must be ensured that the
         process binary and all its auxiliary files are available in
-        the <function>chroot()</function> jail.</para></listitem>
+        the <function>chroot()</function> jail. Note that setting this
+        parameter might result in additional dependencies to be added
+        to the unit (see above).</para></listitem>
       </varlistentry>
 
       <varlistentry>
@@ -351,6 +380,7 @@
         <para>This setting defaults to
         <option>null</option>.</para></listitem>
       </varlistentry>
+
       <varlistentry>
         <term><varname>StandardOutput=</varname></term>
         <listitem><para>Controls where file descriptor 1 (STDOUT) of
@@ -416,8 +446,11 @@
         <para>This setting defaults to the value set with
         <option>DefaultStandardOutput=</option> in
         <citerefentry><refentrytitle>systemd-system.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
-        which defaults to <option>journal</option>.</para></listitem>
+        which defaults to <option>journal</option>. Note that setting
+        this parameter might result in additional dependencies to be
+        added to the unit (see above).</para></listitem>
       </varlistentry>
+
       <varlistentry>
         <term><varname>StandardError=</varname></term>
         <listitem><para>Controls where file descriptor 2 (STDERR) of
@@ -428,8 +461,11 @@
         standard error. This setting defaults to the value set with
         <option>DefaultStandardError=</option> in
         <citerefentry><refentrytitle>systemd-system.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
-        which defaults to <option>inherit</option>.</para></listitem>
+        which defaults to <option>inherit</option>. Note that setting
+        this parameter might result in additional dependencies to be
+        added to the unit (see above).</para></listitem>
       </varlistentry>
+
       <varlistentry>
         <term><varname>TTYPath=</varname></term>
         <listitem><para>Sets the terminal device node to use if