"switch" subscribes to an Observable that emits Observables, also known as a higher-order Observable.
解释： “ that emits Observables” 作为前面 “an Observable” 的修饰从句。Each time it observes one of these emitted inner Observables,
the output Observable subscribes to the inner Observable and begins emitting the items emitted by that.
解释： “the output Observable subscribes to the inner Observable and begins emitting the items emitted by that.” 补全的写法应该是：
“the output Observable subscribes to the inner Observable and (the output) begins emitting the items emitted by (the inner Observable).” 去掉的“that”其实就是“the inner Observable”Returns an Observable that emits items based on applying a function that you supply to each item emitted by the source Observable, where that function returns an (so-called "inner") Observable.
分三个阶段来看：
第一阶段：Returns an Observable that (1. blah, blah, blah....) ;
解释： 返回一个Observable
第二阶段：Returns an Observable that emits items (2. blah, blah, blah....) emitted by the source Observable
解释：返回一个会emits items的Observable。
这些 items 是 emited 自 the source Observable 的。
（说明是两层Observable, 即 higher-order Observable）
第三阶段：(2. blah, blah, blah....) 所指代的那一段：based on applying a function that you supply to each item
解释：第二阶段中 每个 item 都是由你所提供的（自定义） function 加工过的
