Logging documentation should explain InitializeLog and SetStderrThreshold

### Describe the issue

I developed a C++ service that uses Abseil logging and I could not figure out why my log messages with INFO severity did not appear in the Linux systemd journal. I reviewed https://abseil.io/docs/cpp/guides/logging and its information about log severity, and I tried defining ABSL_MIN_LOG_LEVEL to 0, but it did not help.

I had previously added a call to `absl::InitializeLog()` (which is not explained in that docs page) in order to eliminate this warning from my server's output: `WARNING: All log messages before absl::InitializeLog() is called are written to STDERR`. I eventually figured out to my dismay that removing this call to `InitializeLog()` was all I needed to fix my problem: INFO log messages now appeared in the journal output.

I eventually stumbled upon an [example](https://github.com/google/sandboxed-api/blob/main/sandboxed_api/examples/zlib/main_zlib.cc#L43) that showed me how to properly initialize Abseil logging by calling `absl::SetStderrThreshold()` in addition to `InitializeLog()`. That led me to [globals.h](https://github.com/abseil/abseil-cpp/blob/master/absl/log/globals.h#L91) which also explains that `--minloglevel` can be used to control this behavior.

I feel misled since Abseil *told me* to call `InitializeLog()`, but it didn't tell me that I might also need to call `SetStderrThreshold()`. I did the right thing and read the documentation, but none of `InitializeLog()`, `SetStderrThreshold()` or `--minloglevel` are mentioned at all at https://abseil.io/docs/cpp/guides/logging. It would be great if this documentation could be improved, it would have saved me quite a bit of troubleshooting.

### Steps to reproduce the problem

Write a C++ service with a main that begins with:

```
int main(int argc, char **argv) {
absl::InitializeLog();
absl::ParseCommandLine(argc, argv);
```

Add some `LOG(INFO)` and `LOG(ERROR)` messages in the service. Build it and register the service with systemd on a Linux system. Start the service then run `journalctl -u `. Observe that the log messages with INFO severity do not appear in the journal, but the messages with ERROR severity do.

Add a call to `absl::SetStderrThreshold(absl::LogSeverityAtLeast::kInfo);` at the start of `main()`, rebuild and restart the service and observe that now INFO log messages appear in the journal.

### What version of Abseil are you using?

https://github.com/abseil/abseil-cpp/archive/1adf896ec842bd9788a1bbede94a33e1402b8ecb.tar.gz

### What operating system and version are you using?

Linux ARM64

### What compiler and version are you using?

gcc

### What build system are you using?

Bazel 6.0.0

### Additional context

_No response_