io: use SeekStart, SeekCurrent, and SeekEnd in io.Seeker documentation
io: use SeekStart, SeekCurrent, and SeekEnd in io.Seeker documentation

The documentation previously used C style enumerations: 0, 1, 2.
While this is pretty much universally correct, it does not help a user
become aware of the existence of the SeekStart, SeekCurrent, and SeekEnd
constants. Thus, we should use them in the documentation to direct people's
attention to them.

Updates #6885

Change-Id: I44b5e78d41601c68a0a1c96428c853df53981d52
Reviewed-on: https://go-review.googlesource.com/23551
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

ImJasonH/go,suncycheng/go,4ad/go,theif519/go,goodlang/good,nawawi/go,akutz/go,akutz/go,goodlang/good,theif519/go,vsekhar/elastic-go,akutz/go,CAFxX/go,Cofyc/go,vsekhar/elastic-go,momchil-velikov/go,polyverse-security/go,polyverse-security/go,derekmarcotte/go,CAFxX/go,linux-on-ibm-z/go,derekmarcotte/go,cloudflare/go,ImJasonH/go,suncycheng/go,4ad/go,ImJasonH/go,momchil-velikov/go,linux-on-ibm-z/go,derekmarcotte/go,CAFxX/go,Cofyc/go,derekmarcotte/go,derekmarcotte/go,theif519/go,vsekhar/elastic-go,ImJasonH/go,vsekhar/elastic-go,momchil-velikov/go,4ad/go,momchil-velikov/go,momchil-velikov/go,ImJasonH/go,akutz/go,4ad/go,suncycheng/go,goodlang/good,4ad/go,Cofyc/go,CAFxX/go,akutz/go,vsekhar/elastic-go,ImJasonH/go,cloudflare/go,cloudflare/go,codestation/go,4ad/go,akutz/go,Cofyc/go,josharian/go,linux-on-ibm-z/go,polyverse-security/go,suncycheng/go,goodlang/good,vsekhar/elastic-go,4ad/go,theif519/go,nawawi/go,codestation/go,vsekhar/elastic-go,Cofyc/go,cloudflare/go,Cofyc/go,linux-on-ibm-z/go,derekmarcotte/go,Cofyc/go,ImJasonH/go,4ad/go,suncycheng/go,golang/go,goodlang/good,derekmarcotte/go,suncycheng/go,polyverse-security/go,CAFxX/go,cloudflare/go,codestation/go,josharian/go,cloudflare/go,CAFxX/go,golang/go,goodlang/good,linux-on-ibm-z/go,linux-on-ibm-z/go,CAFxX/go,nawawi/go,suncycheng/go,nawawi/go,josharian/go,nawawi/go,polyverse-security/go,josharian/go,josharian/go,polyverse-security/go,codestation/go,codestation/go,momchil-velikov/go,vsekhar/elastic-go,akutz/go,goodlang/good,derekmarcotte/go,codestation/go,josharian/go,theif519/go,theif519/go,linux-on-ibm-z/go,codestation/go,nawawi/go,suncycheng/go,polyverse-security/go,momchil-velikov/go,theif519/go,golang/go,golang/go,nawawi/go,golang/go,goodlang/good,cloudflare/go,golang/go,CAFxX/go,akutz/go,linux-on-ibm-z/go,josharian/go,ImJasonH/go,golang/go,Cofyc/go,codestation/go,theif519/go,cloudflare/go,christopher-henderson/Go,polyverse-security/go,nawawi/go,josharian/go,golang/go,momchil-velikov/go