doc: Document reopen of output file via SIGHUP in pg_recvlogical.

doc: Document reopen of output file via SIGHUP in pg_recvlogical.

When pg_recvlogical receives a SIGHUP signal, it closes the current
output file and reopens a new one. This is useful since it allows us to
rotate the output file by renaming the current file and sending a SIGHUP.

This behavior was previously undocumented. This commit adds
the missing documentation.

Back-patch to all supported versions.

Author: Fujii Masao <masao.fujii@gmail.com>
Reviewed-by: Shinya Kato <shinya11.kato@gmail.com>
Discussion: http://postgr.es/m/0977fc4f-1523-4ecd-8a0e-391af4976367@oss.nttdata.com
Backpatch-through: 13