regexp: document that it is linear in the input size. Fixes issue 7488.
regexp: document that it is linear in the input size.
Fixes issue 7488.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://codereview.appspot.com/78050043

rdp/rogerpack2005-golang,bryanxu/go-zh,d0f/go-zh,sanjosh/sanjos100-tipc,bryanxu/go-zh,rdp/rogerpack2005-golang,webfd/go-zh,rdp/rogerpack2005-golang,sanjosh/sanjos100-tipc,bryanxu/go-zh,bryanxu/go-zh,webfd/go-zh,d0f/go-zh,rdp/rogerpack2005-golang,bryanxu/go-zh,bryanxu/go-zh,bryanxu/go-zh,rdp/rogerpack2005-golang,d0f/go-zh,rdp/rogerpack2005-golang,sanjosh/sanjos100-tipc,d0f/go-zh,webfd/go-zh,webfd/go-zh,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,d0f/go-zh,bryanxu/go-zh,webfd/go-zh,webfd/go-zh,d0f/go-zh,sanjosh/sanjos100-tipc,d0f/go-zh,webfd/go-zh,d0f/go-zh,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang,sanjosh/sanjos100-tipc,webfd/go-zh,rdp/rogerpack2005-golang