Improve documentation for wal-g HTTP proxy environment variables

[pchaseh]

This stems from a problem that has personally stung me when using spilo and migrating to an architecture that incorporates S3. By setting uppercase HTTP{S}_PROXY environment variables in the Spilo docker container I experienced a problem where my archive_command command was failing, resulting in the inability to push WAL segments to my S3 bucket (which was only accessible over an HTTP proxy). Despite this, running the exact same command manually from a shell instance inside of the container (wal-g wal-push) or the postgres_backup_script.sh script worked just fine.

It took me reading through configure_spilo.py to determine that, while other components would function just fine with these uppercase environment variables, wal-g was failing when executed within specific contexts because Spilo was not forwarding them to my wal-e.d/env folder.

While environment variables on Linux are evidently case sensitive, I believe that having this note in the documentation could be a net positive, especially given scenarios where the behaviour is not consistent which makes troubleshooting far harder.