Why just put it in the documentation? Documentation is liable to drift from implementation, and AFAIK no compiler or runtime verifies the accuracy of comments. I'd feel much better about adding asserts to the original code, leaving it for a few generations of testing and exposure, and then eventually remove the conditionals. The assert calls then function as executable documentation.

Depending on your user-base (i.e. if you distribute headers to other developers with precompiled code), the documentation may be necessary on its own... but it's much weaker than an assert.