docs: Clarified how to find out when a switch is toggled

Per Chris, the existing doc was incorrect: you *can't* check for a
button press and a switch flip the same way. (The button triggers a
click event, but the switch does not.) Chris suggested that we just
remove the reference to onClick and suggest using a listener for
both kinds of buttons (ToggleButton and Switch).

Also pulled one note out of a section where it didn't fit (the bit
about changing a button/switch's state programmatically didn't have
much to do with listening for clicks) and put it at the top, and I
fixed a Javadoc typo for a relevant class that I happened to notice.

See first comment for doc stage location.

bug: 20625504
Change-Id: I9c8975111381e5b169f6a61454ef3a93da635759