doc/go1.18: minor tweaks to generics section
doc/go1.18: minor tweaks to generics section

This CL reorders the bullet points in the generics section to more
closely match what I think users will consider most important. I put
the ~ token before the mention of ~T in interfaces to avoid a forward
reference, though I wonder if we actually want to spent a couple more
sentences saying what union and ~T types are, since most people are
going to care about that a lot more than they care about the low-level
detail that there's a new token.

For #47694.

Change-Id: Ib84f096ef6346a711801268ce362b64fa423d3f2
Reviewed-on: https://go-review.googlesource.com/c/go/+/369734
Trust: Austin Clements <7ea35d812706d9213868749011af1ed4fa2f6aa0@google.com>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/go,golang/go,cloudflare/go,golang/go,golang/go,cloudflare/go,CAFxX/go,nawawi/go,CAFxX/go,CAFxX/go,nawawi/go,cloudflare/go,CAFxX/go,nawawi/go,cloudflare/go,cloudflare/go,nawawi/go,golang/go,nawawi/go,nawawi/go,cloudflare/go,nawawi/go,golang/go,nawawi/go,golang/go,cloudflare/go,CAFxX/go,golang/go,CAFxX/go,cloudflare/go,CAFxX/go,CAFxX/go