text/scanner: clarify documentation on Pos and Position
text/scanner: clarify documentation on Pos and Position

For #20292. (See discussion in that issue.)

Change-Id: I44cd69394fc47a01776905ec34305ba524c89883
Reviewed-on: https://go-review.googlesource.com/43452
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

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