man: avoid abbreviated "cgroups" terminology (#4396)

Let's avoid the overly abbreviated "cgroups" terminology. Let's instead write:

"Linux Control Groups (cgroups)" is the long form wherever the term is
introduced in prose. Use "control groups" in the short form wherever the term
is used within brief explanations.

Follow-up to: #4381

man/sd_bus_creds_get_pid.xml

@@ -366,7 +366,7 @@
     -ENXIO is returned.</para>
 
     <para><function>sd_bus_creds_get_cgroup()</function> will retrieve
-    the cgroup path. See <ulink
+    the control group path. See <ulink
     url="https://www.kernel.org/doc/Documentation/cgroups/cgroups.txt">cgroups.txt</ulink>.
     </para>
 

man/systemd.exec.xml

@@ -75,8 +75,7 @@
     [Service], [Socket], [Mount], or [Swap] sections, depending on the
     unit type.</para>
 
-    <para>In addition, options which control resources through cgroups
+    <para>In addition, options which control resources through Linux Control Groups (cgroups) are listed in
-    are listed in
     <citerefentry><refentrytitle>systemd.resource-control</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
     Those options complement options listed here.</para>
   </refsect1>

man/systemd.resource-control.xml

@@ -60,12 +60,10 @@
   <refsect1>
     <title>Description</title>
 
-    <para>Unit configuration files for services, slices, scopes,
+    <para>Unit configuration files for services, slices, scopes, sockets, mount points, and swap devices share a subset
-    sockets, mount points, and swap devices share a subset of
+    of configuration options for resource control of spawned processes. Internally, this relies on the Linux Control
-    configuration options for resource control of spawned
+    Groups (cgroups) kernel concept for organizing processes in a hierarchical tree of named groups for the purpose of
-    processes. Internally, this relies on the Control Groups
+    resource management.</para>
-    kernel concept for organizing processes in a hierarchical tree of
-    named groups for the purpose of resource management.</para>
 
     <para>This man page lists the configuration options shared by
     those six unit types. See
@@ -116,7 +114,7 @@
           <term><option>CPU</option></term>
           <listitem>
             <para>Due to the lack of consensus in the kernel community, the CPU controller support on the unified
-            cgroup hierarchy requires out-of-tree kernel patches. See <ulink
+            control group hierarchy requires out-of-tree kernel patches. See <ulink
             url="https://git.kernel.org/cgit/linux/kernel/git/tj/cgroup.git/tree/Documentation/cgroup-v2-cpu.txt?h=cgroup-v2-cpu">cgroup-v2-cpu.txt</ulink>.</para>
 
             <para><varname>CPUWeight=</varname> and <varname>StartupCPUWeight=</varname> replace

man/systemd.unit.xml

@@ -1252,7 +1252,7 @@
           <row>
       <entry><literal>%r</literal></entry>
       <entry>Control group path of the slice the unit is placed in</entry>
-      <entry>This usually maps to the parent cgroup path of <literal>%c</literal>.</entry>
+      <entry>This usually maps to the parent control group path of <literal>%c</literal>.</entry>
           </row>
           <row>
       <entry><literal>%R</literal></entry>