regexp: document that Longest method is not concurrent-safe
regexp: document that Longest method is not concurrent-safe

Change-Id: I9ec137502353e65325087dfb60ee9bd68ffd286d
Reviewed-on: https://go-review.googlesource.com/38447
Reviewed-by: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

vsekhar/elastic-go,vsekhar/elastic-go,akutz/go,josharian/go,ImJasonH/go,cloudflare/go,cloudflare/go,polyverse-security/go,derekmarcotte/go,golang/go,vsekhar/elastic-go,ImJasonH/go,Cofyc/go,nawawi/go,codestation/go,nawawi/go,Cofyc/go,josharian/go,cloudflare/go,christopher-henderson/Go,golang/go,codestation/go,akutz/go,codestation/go,akutz/go,golang/go,josharian/go,ImJasonH/go,nawawi/go,Cofyc/go,polyverse-security/go,nawawi/go,derekmarcotte/go,polyverse-security/go,josharian/go,vsekhar/elastic-go,CAFxX/go,golang/go,ImJasonH/go,polyverse-security/go,codestation/go,akutz/go,derekmarcotte/go,golang/go,ImJasonH/go,cloudflare/go,cloudflare/go,Cofyc/go,ImJasonH/go,polyverse-security/go,josharian/go,josharian/go,akutz/go,akutz/go,CAFxX/go,derekmarcotte/go,CAFxX/go,vsekhar/elastic-go,josharian/go,nawawi/go,akutz/go,cloudflare/go,Cofyc/go,polyverse-security/go,derekmarcotte/go,CAFxX/go,Cofyc/go,golang/go,ImJasonH/go,Cofyc/go,nawawi/go,codestation/go,derekmarcotte/go,golang/go,nawawi/go,vsekhar/elastic-go,codestation/go,cloudflare/go,akutz/go,derekmarcotte/go,cloudflare/go,CAFxX/go,vsekhar/elastic-go,vsekhar/elastic-go,codestation/go,CAFxX/go,codestation/go,polyverse-security/go,CAFxX/go,derekmarcotte/go,CAFxX/go,Cofyc/go,polyverse-security/go,ImJasonH/go,golang/go,josharian/go,nawawi/go