flag: document use of FlagSet name parameter
flag: document use of FlagSet name parameter

Fixes #24225

Change-Id: I876ac1b9d3615283f7b88cfa0b965ef81a57f056
Reviewed-on: https://go-review.googlesource.com/102955
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

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