Fix docs for __android_log_is_loggable.

len is not a parameter for the function this comment was attached to.
It seems to be that the documentation was meant to apply to both
functions, so copy it and remove the incorrect parameter from the one
that does not take it.

Test: build_ndk_docs.py
Bug: None
Change-Id: I97225d4ba925a33c73b6c7f81bb7b18def363489