doc: update Implementation and Performance sections of the FAQ
doc: update Implementation and Performance sections of the FAQ

Changes are mostly about making more about now than about the past,
changing some verb tenses, and mentioning gollvm (which should
be pronounced "gollum" if you ask me).

Update #26107

Change-Id: I6c14f42b9fc2684259d4ba8bc149d7ec9bb83d15
Reviewed-on: https://go-review.googlesource.com/123917
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

josharian/go,josharian/go,nawawi/go,cloudflare/go,CAFxX/go,josharian/go,nawawi/go,CAFxX/go,golang/go,akutz/go,akutz/go,cloudflare/go,cloudflare/go,josharian/go,cloudflare/go,josharian/go,CAFxX/go,CAFxX/go,akutz/go,cloudflare/go,cloudflare/go,nawawi/go,golang/go,nawawi/go,josharian/go,golang/go,CAFxX/go,CAFxX/go,josharian/go,akutz/go,akutz/go,nawawi/go,golang/go,nawawi/go,akutz/go,golang/go,josharian/go,CAFxX/go,golang/go,akutz/go,cloudflare/go,nawawi/go,akutz/go,CAFxX/go,cloudflare/go,golang/go,golang/go,nawawi/go