Improve documentation by link getUpstreamCheckerNames and getSuppressWarningsPrefixes

[Ao-senXiong]

Fixes #1011

[wmdietl]

I think the point of #1011 was to improve the documentation of these methods in BaseTypeChecker. That documentation should make clear that these two methods likely want similar behavior.
The improvement here might be good, but please also look at BaseTypeChecker (and also check whether these methods are overridden elsewhere).

[Ao-senXiong]

Thanks! Since those two methods are in sourcechecker, I noticed there are five overrides for getSuppressWarningsPrefixes in FenumChecker, InitializationChecker, NullnessNonInitSubChecker, SubtypingChecker and UnitsChecker. While
getUpstreamCheckerNames only have two overrides in InitializationChecker and InitializationFieldAccessChecker.