Operations: Don't imply an operation is straightforwardly useable (#404)

The doc says "[An operation] can be used like use a command line tool or
an expression", which may be read as "you can use an (abstract)
operation just like how you use a (concrete) command line tool or
expression". This is of course false and directly contradicted by the
next example.

To avoid this implication, reword it as "it can appear...".

Also, a minor clarification about "Sphinx Graphviz", which referred to a
specific processing directive in the Sphinx doc.

src/topics/operations.md

@@ -13,16 +13,17 @@ you are ready to submit the workflow to a CWL runner:
 :caption: "`operations.cwl`"
 ```
 
-The `uppercase` step of the workflow is an operation. It can be used like
-a command line tool or an expression. You can also plot it with the
+The `uppercase` step of the workflow is an operation. It can appear where a
+command line tool or an expression is expected. You can also plot it with the
 CWL Viewer or `cwltool`:
 
 ```{runcmd} cwltool --print-dot operations.cwl
 :working-directory: src/_includes/cwl/operations/
 ```
 
-The output of the command above can be rendered with a Graphviz renderer. The following
-image is rendered with the Sphinx Graphviz directive (this user guide is built with Sphinx):
+The output of the command above can be rendered with a Graphviz renderer. The
+following image is rendered with the Sphinx `{graphviz}` directive (this user
+guide is built with Sphinx):
 
 ```{graphviz}