Usage Messages of built-in functions have embedded in-line formatting. For example:
In[1]:= ActionMenu::usage // FullForm
Out[1]//FullForm= 
"\!\(\*RowBox[{\"ActionMenu\", \"[\", RowBox[{StyleBox[\"name\", \"TI\
\"], \",\", RowBox[{\"{\", \
RowBox[{RowBox[{SubscriptBox[StyleBox[\"lbl\", \"TI\"], \
StyleBox[\"1\", \"TR\"]], \":>\", SubscriptBox[StyleBox[\"act\", \"TI\
\"], StyleBox[\"1\", \"TR\"]]}], \",\", \
RowBox[{SubscriptBox[StyleBox[\"lbl\", \"TI\"], StyleBox[\"2\", \
\"TR\"]], \":>\", SubscriptBox[StyleBox[\"act\", \"TI\"], \
StyleBox[\"2\", \"TR\"]]}], \",\", StyleBox[\"\[Ellipsis]\", \
\"TR\"]}], \"}\"}]}], \"]\"}]\) represents an action menu with label \
\!\(\*StyleBox[\"name\", \"TI\"]\), and with items labeled \
\!\(\*SubscriptBox[StyleBox[\"lbl\", \"TI\"], StyleBox[\"i\", \
\"TI\"]]\), that evaluates the expression \
\!\(\*SubscriptBox[StyleBox[\"act\", \"TI\"], StyleBox[\"i\", \
\"TI\"]]\) if the corresponding item is chosen."
One can see that this in-line formatting is based on the set of styles defined in "Styles for Inline Formatting" section of the Core.nb stylesheet. But I have not found any documentation for these styles as well as any description of the convenient algorithm of adding formatting to usage Messages.
What is the convenient way to add in-line formatting to user-defined usage Messages in Mathematica? What are the usage rules of default styles for in-line formatting defined in the Core.nb stylesheet? I would like to add in-line formatting to the usage Messages in my package just with Mathematica, without installing additional components like Workbench etc.
P.S. The syntax of the embedded in-line formatting in Strings is partially documented on the tutorial page "String Representation of Boxes." Related question in the official newsgroup on this syntax: "(any documentation for) linear syntax?" Displaying of such strings in the FrontEnd is controlled by the option ShowStringCharacters->False of Cell.
 
     
     
 

 
    