Add a section on lambda's.

Dissuade their use when named function is better.
Prohibit quoting.
Move existant lambda information into section.

This will conflict with #12 . I'll rebase locally if #12 is merged here.
What's the rationale for this one?  This example doesn't really follow from the others (it doesn't have the same issues as hooks), and there are some examples of built-in `defcustom`s being set to lambdas (one example is `ibuffer-maybe-show-predicates`). Also, since the variable is named, I don't really see a maintainability issue.