Documentation updates.

README

src/lib-sieve/ext-fileinto.c

src/lib-sieve/ext-reject.c

src/lib-sieve/plugins/vacation/ext-vacation.c

src/lib-sieve/sieve.c

src/lib-sieve/sieve.h

src/plugins/lda-sieve/lda-sieve-plugin.c

src/sieve-bin/sievec.c