doc: extend breakpointHook documentation

doc/stdenv.xml

@@ -2451,16 +2451,19 @@ addEnvHooks "$hostOffset" myBashFunction
      </term>
      <listitem>
       <para>
-       This hook will make a build pause instead of stopping
-       when a failure happen. It prevents nix to cleanup the build
-       environment immediatly and allows the user to attach
-       to a build environemnt using the <varname>cntr</varname> command.
-       On build error it will print the instruction that are neccessary for cntr.
-       Note that <varname>cntr</varname> is not installed by default and
-       needs to be installed seperatly. <varname>cntr</varname> also needs to be executed
-       on the machine that is doing the build, which might be not the case
-       when remote builders are enabled. <varname>cntr</varname> is only supported
-       on linux based platforms.
+       This hook will make a build pause instead of stopping when a failure
+       happen. It prevents nix to cleanup the build environment immediatly and
+       allows the user to attach to a build environment using the
+       <command>cntr</command> command. On build error it will print the
+       instruction that are neccessary for <command>cntr</command>. Installing
+       cntr and running the command will provide shell access to the build
+       sandbox of failed build. At <filename>/var/lib/cntr</filename> the
+       sandbox filesystem is mounted. All commands and files of the system are
+       still accessible within the shell. To execute commands from the sandbox
+       use the cntr exec subcommand. Note that <command>cntr</command> also
+       needs to be executed on the machine that is doing the build, which might
+       be not the case when remote builders are enabled.
+       <command>cntr</command> is only supported on linux based platforms.
       </para>
      </listitem>
     </varlistentry>