Add missing information on underscore usage to the book

[SplittyDev]

I noticed that there is virtually no information on the usage of underscores in numeric literals in the book.

The syntax index only mentions underscores in the context of pattern matching as "ignored" pattern binding, so I suggest adding the other use of underscores to the syntax index too.

[steveklabnik]

I would be willing to mentor anyone who wants to help with this issue.

[abhijeetbhagat]

I can update.

[abhijeetbhagat]

@SplittyDev btw there is text about using underscores in integer literals here and here

[kyrias]

Is #34990 enough for this, or do you want something more in-depth?

[pmatos]

It seems #34990 was merged, should this be closed?

[steveklabnik]

I believe so!