docs(error/unpr): add hint about using `ngStrictDi`

The unknown provider error often happens when code is minified and one
did not use the correct syntax that supports minification. It's
frustrating to have to hunt for a bug in minified code, so adding the
simple hint that `ngStrictDi` will tell you what is wrong in the original
code will save you quite some trouble.

Closes #12717