docs: AutomaticEnv probes every key, and loads nothing

[ump45nose]

Overview:

Corrects the AutomaticEnv godoc comment (both the package-level and the *Viper method copy) so it describes what find() actually does.

What problem does it solve?:

The comment makes two claims that the code contradicts:

1. "check if environment variables match any of the existing keys (config, default or flags)" — the probe is not limited to keys that already exist. find() builds the variable name from whatever key is being looked up, so any Get/GetString/IsSet call hits the environment:

   // Env override next
   if v.automaticEnvApplied {
       envKey := strings.Join(append(v.parents, lcaseKey), ".")
       // even if it hasn't been registered, if automaticEnv is used,
       // check any Get request
       if val, ok := v.getEnv(v.mergeWithEnvPrefix(envKey)); ok {

   (viper.go:1271-1278, and the inline comment says the opposite of the doc comment.)

2. "they are loaded into Viper" — nothing is loaded. AutomaticEnv() only flips a flag (viper.go:1460-1462), and getEnv calls os.LookupEnv on each lookup (viper.go:416-424). v.env is written only by BindEnv (viper.go:1163-1165), which is why AllKeys() — and therefore AllSettings() and WriteConfig() — never sees a key that exists only in the environment (viper.go:1980-1997).

The new wording also states how the variable name is derived, since that is what a user actually needs: the key is upper-cased by mergeWithEnvPrefix, prefixed with the SetEnvPrefix value when set (viper.go:389-395), and passed through the SetEnvKeyReplacer value in getEnv when set.

No behaviour change, no API change — comment text only.

Special notes for a reviewer:

• Testing boundary: this session could not run go build, go test, or gofmt (the sandbox refused them), so the change is reviewable on the cited lines alone. It is a comment edit; CI is the real check.
• Checked against open PRs before filing: no open PR touches this comment. gh search prs --repo spf13/viper AutomaticEnv --state open returns only behaviour/feature PRs (Fix #1732 AutomaticEnv does not override Unmarshaled config slice values #1733, Add support for slices when using environement variables #1785, fix: apply envPrefix to all BindEnv variable names #1874, feat: handle other options for prefix #1719, fix: capitalize the second param in BindEnv #1397, Add example regarding required flags with Cobra + Viper #852), and the doc PRs I could see (doc: fix inverted wrap direction on FileNotFoundFromSearchError #2164, doc: DecodeHook's stated default is the hook the code replaced #2165, docs: clarify extensionless config handling and SetConfigType usage #2155, docs: fix typos in README.md and UPGRADE.md #2150, Fix "This is error wraps" doc typo on ConfigFileNotFoundError #2130) edit errors.go, viper.go:60, and the README/UPGRADE instead.
• Deliberately left alone: README.md:259-260 says "search for any environment variable prefixed and load them in", which has the same "load" imprecision. Happy to reword it in this PR if maintainers want the README synced too.
• AI assistance disclosure: prepared with an AI coding agent (per the repo's expectation of transparency); the claims were verified by reading viper.go line by line, and I am responsible for the result.