Address review comments

doc/environment_variables.md

gRPC-Core.podspec

include/grpc/impl/codegen/port_platform.h

src/core/ext/client_channel/resolver_registry.c

src/core/ext/client_channel/resolver_registry.h

src/core/ext/resolver/dns/c_ares/dns_resolver_ares.c

src/core/ext/resolver/dns/c_ares/grpc_ares_ev_driver.h

src/core/ext/resolver/dns/c_ares/grpc_ares_ev_driver_posix.c

src/core/ext/resolver/dns/c_ares/grpc_ares_wrapper.c

src/core/ext/resolver/dns/c_ares/grpc_ares_wrapper.h

src/core/ext/resolver/dns/native/dns_resolver.c

templates/gRPC-Core.podspec.template

...client_channel/resolvers/dns_resolver_connectivity_test.c

test/core/end2end/fuzzers/api_fuzzer.c

test/core/end2end/goaway_server_test.c