design/19308-number-literals.md: document 'i' suffix on any number literal
design/19308-number-literals.md: document 'i' suffix on any number literal

Document that the imaginary suffix 'i' may now be used on any (non-imaginary)
number literal, while maintaining backward compatibility.

While at it, consolidate the syntax around for int_lit and float_lit
to match the respective spec changes in https://golang.org/cl161098,
and clean up order of examples.

Change-Id: Ia6f2e3c5c0b7bcb70f81042ed20b2b18ecb81e6e
Reviewed-on: https://go-review.googlesource.com/c/proposal/+/163857
Reviewed-by: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

golang/proposal,golang/proposal