reflect: document that value.NumMethod counts exported methods
reflect: document that value.NumMethod counts exported methods

Updates #17686
Fixes #20848

Change-Id: I35d58c7d1aa74d3e7867124070e27c787d444b04
Reviewed-on: https://go-review.googlesource.com/47210
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

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