doc: add note about invalid flag errors to 1.10 release notes
doc: add note about invalid flag errors to 1.10 release notes

Updates #23672
Updates #23749

Change-Id: I85b6493bd1e4095398508b7ef395c35463dfbde1
Reviewed-on: https://go-review.googlesource.com/94158
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Reviewed-by: Andrew Bonventre <682ad9f487f614f19e8b7cc5d340bdc8e1c1b128@golang.org>

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