sync.once: document that Do cannot be invoked recursively. documentation change only.
sync.once: document that Do cannot be invoked recursively.
documentation change only.

R=rsc
CC=golang-dev
http://codereview.appspot.com/1964043

dsriram/golang,h8liu/golang,dsriram/golang,pombredanne/go-deleteme,apparentlymart/golang,dsriram/golang,h8liu/golang,apparentlymart/golang,h8liu/golang,dsriram/golang,ssrl/go,dsriram/golang,h8liu/golang,tv42/old-go,ssrl/go,gnanderson/go,pombredanne/go-deleteme,gnanderson/go,apparentlymart/golang,apparentlymart/golang,gnanderson/go,dsriram/golang,apparentlymart/golang,h8liu/golang,gnanderson/go,ssrl/go,h8liu/golang,ssrl/go,h8liu/golang,apparentlymart/golang,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,gnanderson/go,dsriram/golang,apparentlymart/golang,tv42/old-go,ssrl/go,ssrl/go,gnanderson/go,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,gnanderson/go,ssrl/go,tv42/old-go