clarify mutex apis
clarify mutex apis

Change-Id: Ia5926e67001519019ceda92ee8c6fae446cbc077
GitOrigin-RevId: 403cbcbb12a7d19614805a261cfadc64394d1243

google-coral/coralmicro,google-coral/coralmicro,google-coral/coralmicro,google-coral/coralmicro,google-coral/coralmicro