Document how the "_" pattern works

[hanna-kruppe]

#31153 was closed and the user who filed it was educated, but they're not the first and not the last person to be confused by this. AFAIK the behavior of _ — especially how it's different from _foo — is not documented, at least I couldn't find anything in the reference and it seems a tad obscure to be mentioned in the book.

[durka]

Well it is covered here (and linked from the Syntax Index) but it doesn't explain the behavior discussed in #31153.