Reintroduce setConnectTimeout() warning lost in Android N.

In Android N, a warning in the documentation of
URLConnection.setConnectTimeout(int) was accidentally
lost. This CL reintroduces it. The documentation is slightly
tweaked to refer specifically to the default implementation
of HttpURLConnection, rather than "this client". Still, the
documentation is left in URLConnection.java because moving
it to HttpURLConnection.java would require an override of
setConnectTimeout(int).

Bug: 63124970
Test: Treehugger
Change-Id: I27f4ad953dbe07c1993b01560a826ed60db174ca