[ruby-core:28158] Documentation for const_missing

Hello!

I came upon this while answering Yehuda's const_missing proposal. The
current documentation for the method seems odd. Can we rewrite the
following rant in the documentation of const_missing?

     [...] The following code is a (very
     bad) example: if reference is made to an undefined constant, it
     attempts to load a file whose name is the lowercase version of the
     constant (thus class +Fred+ is assumed to be in file +fred.rb+). If
     found, it returns the value of the loaded class. It therefore
     implements a perverse kind of autoload facility.

It's not perverse, it's useful. If it really was a bad code example, it
should not be in the documentation at all (unless we want to say that
using const_missing is generally discouraged?). So, here's a more
neutral version, removing "(very bad)" and "perverse".

Patch attached (for trunk); earlier Ruby versions are affected as well.
[murphy]