dtls: update some code documentation
dtls: update some code documentation

This adds some new and updates some old code documentation.

Signed-off-by: Hauke Mehrtens <435ddd46dc66c007a1fa20144c823d37e0d23436@hauke-m.de>

jvermillard/tinydtls,fvdnabee/tinydtls,yunus/TinyDTLS-WebID,spark/tinydtls,nmacs/tinydtls,fvdnabee/tinydtls,spark/tinydtls,nmacs/tinydtls,fvdnabee/tinydtls,spark/tinydtls,OlegHahm/tinydtls,nmacs/tinydtls,thecodemaiden/tinydtls-fork,nmacs/tinydtls,OlegHahm/tinydtls,OlegHahm/tinydtls,thecodemaiden/tinydtls-fork,thecodemaiden/tinydtls-fork,yunus/TinyDTLS-WebID,jvermillard/tinydtls,jvermillard/tinydtls,yunus/TinyDTLS-WebID