Document -AenableReturnsReceiverForRlc

[Calvin-L]

PR #6838 disabled the returns-receiver checker by default in the resource leak checker, which was certainly a good change. However, the manual still suggests (now incorrectly) that the returns-receiver checker has to be disabled if you don't want it:

checker-framework/docs/manual/called-methods-checker.tex

Line 56 in af87b07

\item The \<-ACalledMethodsChecker\_disableReturnsReceiver> option disables

The replacement option -AenableReturnsReceiverForRlc is not documented.