doc/go1.18: document type parameter name restriction
doc/go1.18: document type parameter name restriction

For #47694.

Change-Id: I00862f987a0ff9f71e0295ce4320e6f9a6a4332f
Reviewed-on: https://go-review.googlesource.com/c/go/+/376414
Trust: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>
Run-TryBot: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

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