This is really a nitpick, but words like "commonly" and "usually" are a (mild) documentation smell to me.  What kind of OpenSSL based TLS application would one not find such a thing in?  If "it" (fewer pronouns would also be better, as would a subject for the sentence fragment "Commonly found…") is "usually" found in PEM format, where else might it be found?  Also, if the only method is called `load_pem`, then presumably it is *always* found in PEM format, although other comments on this PR suggest to me that in fact it supports DER format in some way too (or was that just referring to the underlying API?)