add error case in doc for Index. simplify code slightly.
add error case in doc for Index. simplify code slightly.

R=rsc
DELTA=5  (1 added, 0 deleted, 4 changed)
OCL=27148
CL=27151

wesolows/golang-old,dsriram/golang,tv42/old-go,wesolows/golang,firebitsbr/golang-old,tianon/debian-golang,firebitsbr/golang-old,tianon/debian-golang,pombredanne/go-deleteme,nightlyone/golang,wesolows/golang,nightlyone/golang,ssrl/go,dsriram/golang,wesolows/golang,pombredanne/go-deleteme,tv42/old-go,h8liu/golang,wesolows/golang-old,gnanderson/go,firebitsbr/golang-old,dsriram/golang,ssrl/go,bdwalton/debian-golang,nightlyone/golang,bdwalton/debian-golang,wesolows/golang-old,gnanderson/go,apparentlymart/golang,tianon/debian-golang,nightlyone/golang,pombredanne/go-deleteme,firebitsbr/golang-old,gnanderson/go,dsriram/golang,tianon/debian-golang,h8liu/golang,ssrl/go,firebitsbr/golang-old,tv42/old-go,h8liu/golang,gnanderson/go,pombredanne/go-deleteme,wesolows/golang-old,wesolows/golang,ssrl/go,dsriram/golang,ssrl/go,apparentlymart/golang,tianon/debian-golang,wesolows/golang-old,h8liu/golang,firebitsbr/golang-old,ssrl/go,tianon/debian-golang,gnanderson/go,gnanderson/go,bdwalton/debian-golang,pombredanne/go-deleteme,wesolows/golang,bdwalton/debian-golang,firebitsbr/golang-old,apparentlymart/golang,tv42/old-go,gnanderson/go,bdwalton/debian-golang,tv42/old-go,wesolows/golang,h8liu/golang,tianon/debian-golang,bdwalton/debian-golang,bdwalton/debian-golang,wesolows/golang-old,ssrl/go,tv42/old-go,apparentlymart/golang,h8liu/golang,nightlyone/golang,dsriram/golang,nightlyone/golang,bdwalton/debian-golang,apparentlymart/golang,apparentlymart/golang,wesolows/golang-old,firebitsbr/golang-old,pombredanne/go-deleteme,tianon/debian-golang,wesolows/golang,apparentlymart/golang,nightlyone/golang,dsriram/golang,h8liu/golang,tv42/old-go,pombredanne/go-deleteme