Update comments for Makefile variables about APEX jars.

Modules available in Q should not add new entries to
PRODUCT_APEX_*_JARS. Otherwise, there will be problems when a Q device
gets new modules and then OTAs to S, in which case, the device will have
the S version of SdkExtension (because SdkExtension is not available in
Q, see go/mainline-ota-problem), and the S version of SdkExtension
cannot consume those new entries correctly.

Bug: 203198541
Test: N/A
Change-Id: I15367ca59a06b4ee5fdb3803f2e4615b5d03b458