How detailed should a methodology be presented?
This is extremely dependent upon the journal you are submitting to (and who the audience of that journal is) and the roll of the dice on who your reviewers will be. Where one reviewer believes that it is critical to demonstrate that you have met your assumptions for regression with the inclusion of residual plots and formal tests, another (and equally as qualified) will state that such inclusions only add clutter to the methods. And finally, you might have an editor come in and say that it should be included but moved as an addendum. Even scanning prior journal articles is not always the best indicator (though its a very good one) because editors change and reviewers are different. 

Your best bet is to go with what you feel is correct and allows for the clearest replication of your methods. If this involves going into the mechanics of back propagation and what a sigmoid is, then so be it. If, as you have stated, that you feel that a reference will suffice, then thats ok as well. 

To give an example, I have two papers coming out this fall where-in both use the same method to calculate a statistic. Paper one, the reviewer felt that I should explain, in detail, how that statistic is calculated due to probable reader unfamiliarity. In paper two, the reviewer accepted the simple citation of "this statistic was calculated in the manner described in _____(XXXX)". 

So, in summation, it depends, and you should use your educated determination as someone with knowledge of the literature to guide you.