Update di-usage.texy

Correct several typos in code examples and also in text (formulation,
typos, etc.)

Done changes?
   - add missing semicolons (code examples)
   - add namespace declaration statements (code examples)
   - add caution block under "Other possibilities" title (text)
   - correct wrong access modifiers on properties/methods (code examples)
   - correct few typos in text (text)
   - place the parameters of a constructor on one single line (code
     examples)