Add Note about unsupported Regex options

[WhatzGames]

As npgsql/efcore.pg#2818 showed, it is not clear on what RegexOptions are actually supported.
I suggest adding a note to the tranlation docs to make it clearer for everyone.

[roji]

Do you think it's really necessary, given that users get a "cannot translate" exception if they try an unsupported version?

In any case, if you think it's important, feel free to submit a PR updating the translations doc page.

[WhatzGames]

I don't think it's an absolute necessity, but the exception is not too clear about why it would fail.
Providing the information about what option is supported or not would at least clarify it.

As soon as I got the time I'll see to add a note.