os/user: document the difference between Username and Name
os/user: document the difference between Username and Name

Fixes #18261.

Change-Id: I4bd7363aac4e62461f61fd95b3c7a18063412182
Reviewed-on: https://go-review.googlesource.com/34241
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

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