Prepare to integrate RxJava 2.x documentation

[DavidMGross]

2.x implementations of RxJava operators & such may be different in important ways from their counterparts in RxJava 1.x, so we need to be ready to be able to distinguish our documentation of 1.x stuff from 2.x stuff.

• https://github.com/ReactiveX/RxJava/wiki/What's-different-in-2.0

[DavidMGross]

For example, accordions that say "RxJava" should probably say "RxJava 1.x" in preparation for 2.x.

See, for instance ReactiveX/RxJava#2780

[DavidMGross]

See ReactiveX/RxJava#3782 and ReactiveX/RxJava#3783 for an overview of some of the changes from 1.x to 2.x

[DavidMGross]

See also ReactiveX/RxJava#4061 (location of 2.x javadocs)

[DavidMGross]

See also ReactiveX/RxJava#4121 and https://github.com/ReactiveX/RxJava/wiki/What's-different-in-2.0-(draft)

[DavidMGross]

See also ReactiveX/RxJava#4153 for a potential reduction in operator variants and the documentation needs this will cause.