spec: document new Go2 number literals

This CL documents the new binary and octal integer literals,
hexadecimal floats, generalized imaginary literals and digit
separators for all number literals in the spec.

Added empty lines between abutting paragraphs in some places
(a more thorough cleanup can be done in a separate CL).

A minor detail: A single 0 was considered an octal zero per the
syntax (decimal integer literals always started with a non-zero
digit). The new octal literal syntax allows 0o and 0O prefixes
and when keeping the respective octal_lit syntax symmetric with
all the others (binary_lit, hex_lit), a single 0 is not automatically
part of it anymore. Rather than complicating the new octal_lit syntax
to include 0 as before, it is simpler (and more natural) to accept
a single 0 as part of a decimal_lit. This is purely a notational
change.

R=Go1.13

Updates #12711.
Updates #19308.
Updates #28493.
Updates #29008.

Change-Id: Ib9fdc6e781f6031cceeed37aaed9d05c7141adec
Reviewed-on: https://go-review.googlesource.com/c/go/+/161098Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>