reflect: adjust documentation on Value
reflect: adjust documentation on Value

Make the documentation more explicit that it is not safe to directly
compare Value. Get straight to the point on how to do it correctly.

Updates #18871

Change-Id: I2aa3253f779636b2f72a1aae8c9bb45d3c32c902
Reviewed-on: https://go-review.googlesource.com/36018
Reviewed-by: Keith Randall <8c99c3a9284e493be632950b84cd789d08ed3e9d@golang.org>

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