doc: update release notes for library support for new literals

[rsc]

The release notes don't mention all the library changes related to the new number literals.
In particular there is new printing in fmt, new parsing in strconv, math/big, and text/scanner.

I started to update the notes but there's a lot to add. Filing so I don't forget.

[odeke-em]

For the checklist, as we update release notes for this issue we can update accordingly here

• textscanner -- completed by CL 183357
• strconv -- completed by CL 191077
• math/big -- completed by CL 191077
• fmt -- completed by CL191578

[gopherbot]

Change https://golang.org/cl/191077 mentions this issue: doc/go1.13: document _ between digits for math/big

[odeke-em]

Alright, all the packages on our checklist have been completed. With that I shall close this issue.

[odeke-em]

I'll reopen this until we have fully verified that fmt's release notes also document the updates; perhaps tonight when I get time.

[gopherbot]

Change https://golang.org/cl/191477 mentions this issue: [release-branch.go1.13] doc/go1.13: document _ between digits for math/big, strconv

[gopherbot]

Change https://golang.org/cl/191578 mentions this issue: doc/go1.13: document fmt's number syntax updates