libbinder: more forcePersist docs

Documenting recommending use for now. Looked at adding a shared library
destructor, but this has limited use when people use _exit, when persist
HALs sometimes exit manually early, and since it only corresponds to
this one mechanism. Given perf team work targetting understanding memory
uses/process which are starting, we can focus/rely on more general
infrastructure.

Fixes: 187946888
Test: N/A
Change-Id: I67c2477bccd9ebc789e20527d5692ea3a107c675