reflect: document DeepEqual(nil map, empty non-nil map) behavior
reflect: document DeepEqual(nil map, empty non-nil map) behavior

Fixes #16531.

Change-Id: I41ec8123f2d3fbe063fd3b09a9366e69722793e5
Reviewed-on: https://go-review.googlesource.com/31355
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

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