[v3] docs patch for concept checkers

[Gabriel Dos Reis]

Phil Edwards <pedwards@disaster.jaj.com> writes:

[...]

| +   <P>Concept checking can be disabled when you build your code.  Just
| +      define (via -D or #define) the macro
| +      <TT>_STL_NO_CONCEPT_CHECKS</TT> (yes, with the leading underscore).
| +      In fact, this may already be defined in the library by default,
| +      depending on what decision we come to.

I'm wondering why it is not the other way around: don't turn on the
current code by default.

-- Gaby