Document what Strict and Lazy mean

[ekmett]

Affects:

Control.Monad.State.Strict
Control.Monad.Writer.Strict
Control.Monad.RWS.Strict

From Ben Gamari:

In general it would be nice if the notion of strictness were better
addressed in the major libraries' documentation. It's a nuanced
issue which is not fully captured by a ".Lazy" or ".Strict" in the
module name.