Replaces the now-incorrect Note at the bottom of c16-proxy ("cpp-httplib
does not read HTTP_PROXY...") with the actual API.
JA is the master per the project's translation workflow; the EN
translation lands in the same PR. Both pages remain `status: "draft"`
for normal review.
Adds two sections:
- Bypass the proxy for specific hosts (set_no_proxy):
pattern syntax, dot-boundary rule, case-insensitivity, IP
normalization via inet_pton, port-specific-entries unsupported,
malformed entries dropped.
- Read proxy settings from the environment (set_proxy_from_env):
which variables are read, lowercase-only http_proxy with an
inline httpoxy / CVE-2016-5385 explanation, threading caveat.
