Add client/server mention in componentDid/WillMount documentation

[vjeux]

Here's the jsfiddle that I used to make sure it is correct: http://jsfiddle.net/vjeux/QeCB5/

[plievone]

Note that at some point React.renderComponentToString behavior differed when DOM was available or not, so there was confusion due to jsfiddles and server experiments. :) Nowadays there are test cases for this: https://github.com/facebook/react/blob/f0eae50/src/browser/server/__tests__/ReactServerRendering-test.js#L150-L159