Make it clear that `workdir' is an option to ShellCommand.

docs/buildbot.texinfo

@@ -5641,15 +5641,6 @@ if you're running a batch file, the error level does not get propagated
 correctly unless you add 'call' before your batch file's name:
 @code{cmd=['call', 'myfile.bat', ...]}.
 
-All ShellCommands are run by default in the ``workdir'', which
-defaults to the ``@file{build}'' subdirectory of the slave builder's
-base directory. The absolute path of the workdir will thus be the
-slave's basedir (set as an option to @code{buildbot create-slave},
-@pxref{Creating a buildslave}) plus the builder's basedir (set in the
-builder's @code{c['builddir']} key in master.cfg) plus the workdir
-itself (a class-level attribute of the BuildFactory, defaults to
-``@file{build}'').
-
 @code{ShellCommand} arguments:
 
 @table @code
@@ -5662,6 +5653,16 @@ string to /bin/sh for interpretation, which raises all sorts of
 difficult questions about how to escape or interpret shell
 metacharacters.
 
+@item workdir
+All ShellCommands are run by default in the ``workdir'', which
+defaults to the ``@file{build}'' subdirectory of the slave builder's
+base directory. The absolute path of the workdir will thus be the
+slave's basedir (set as an option to @code{buildbot create-slave},
+@pxref{Creating a buildslave}) plus the builder's basedir (set in the
+builder's @code{c['builddir']} key in master.cfg) plus the workdir
+itself (a class-level attribute of the BuildFactory, defaults to
+``@file{build}'').
+
 @item env
 a dictionary of environment strings which will be added to the child
 command's environment. For example, to run tests with a different i18n