Clarify some comments around SharedRecoveryState in xlog.c

SharedRecoveryState has been switched from a boolean to an enum as of
commit 4e87c48, but some comments still referred to it as a boolean.

Author: Amul Sul
Reviewed-by: Dilip Kumar, Kyotaro Horiguchi
Discussion: https://postgr.es/m/CAAJ_b97Hf+1SXnm8jySpO+Fhm+-VKFAAce1T_cupUYtnE3Nxig
