reflect: clarify that NumMethod returns only the number of exported methods
reflect: clarify that NumMethod returns only the number of exported methods

Fixes #17686.

Change-Id: I7d07c367e50b448579f9855bea43df76ddb82bd0
Reviewed-on: https://go-review.googlesource.com/32420
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

CAFxX/go,derekmarcotte/go,nawawi/go,suncycheng/go,josharian/go,goodlang/good,linux-on-ibm-z/go,cloudflare/go,nawawi/go,suncycheng/go,Cofyc/go,codestation/go,linux-on-ibm-z/go,akutz/go,josharian/go,derekmarcotte/go,golang/go,linux-on-ibm-z/go,vsekhar/elastic-go,CAFxX/go,nawawi/go,akutz/go,CAFxX/go,ImJasonH/go,linux-on-ibm-z/go,vsekhar/elastic-go,cloudflare/go,josharian/go,linux-on-ibm-z/go,cloudflare/go,ImJasonH/go,polyverse-security/go,codestation/go,codestation/go,nawawi/go,cloudflare/go,vsekhar/elastic-go,golang/go,suncycheng/go,polyverse-security/go,derekmarcotte/go,derekmarcotte/go,nawawi/go,josharian/go,suncycheng/go,christopher-henderson/Go,goodlang/good,polyverse-security/go,josharian/go,linux-on-ibm-z/go,CAFxX/go,ImJasonH/go,golang/go,Cofyc/go,josharian/go,CAFxX/go,CAFxX/go,goodlang/good,goodlang/good,cloudflare/go,ImJasonH/go,akutz/go,golang/go,akutz/go,vsekhar/elastic-go,codestation/go,golang/go,akutz/go,goodlang/good,josharian/go,codestation/go,josharian/go,codestation/go,cloudflare/go,ImJasonH/go,suncycheng/go,goodlang/good,nawawi/go,goodlang/good,nawawi/go,goodlang/good,nawawi/go,akutz/go,derekmarcotte/go,ImJasonH/go,polyverse-security/go,Cofyc/go,polyverse-security/go,vsekhar/elastic-go,vsekhar/elastic-go,akutz/go,akutz/go,Cofyc/go,golang/go,Cofyc/go,cloudflare/go,CAFxX/go,derekmarcotte/go,polyverse-security/go,codestation/go,polyverse-security/go,suncycheng/go,golang/go,golang/go,suncycheng/go,cloudflare/go,ImJasonH/go,codestation/go,linux-on-ibm-z/go,Cofyc/go,derekmarcotte/go,derekmarcotte/go,Cofyc/go,vsekhar/elastic-go,Cofyc/go,linux-on-ibm-z/go,suncycheng/go,polyverse-security/go,CAFxX/go,vsekhar/elastic-go,ImJasonH/go