make clean does not fail when doc does not exist

With the previous version, running make clean without make doc first
resulted in an error on rmdir doc.
Revert "PLayer interface definitions"

This reverts commit b1d22441ef16654a849643b78180a5370a1f2c8d.
Revert "Player.mli : Définition des methodes d'interfaçage"

This reverts commit 9715a20c70b589ba01c64462223f5a002c3594af.
fix fieldgenerator doc
Merge branch 'master' into generation
beaches, beaches!