doc: Add documentation for PGLOADBALANCEHOSTS environment variable in libpq.
Commit
7f5b19817e introduced the PGLOADBALANCEHOSTS environment
variable for connection load balancing in libpq. However, documentation
for this variable was missing.
Author: Fujii Masao
Reviewed-by: Jelte Fennema
Discussion: https://postgr.es/m/
e2b44cf7-aa16-e726-1d4a-
a69508ebf6b9@oss.nttdata.com