Suggested tweaks up to package section.

Major ones:
* Be consistent: "numeric type" and "arithmetic operator".
* if/swtch take "simple statement" not "short variable declaration".
* There was a comment that implied for x,y := range z() might
  evaluate z() multiple times.  I deleted it.

R=r,gri
DELTA=124  (24 added, 4 deleted, 96 changed)
OCL=25706
CL=25715