Document the behavior of android-api-30 feature

Author: Marcin Radomski

README.md

@@ -65,6 +65,48 @@ Therefore this library will only log a warning for subsequent `init_once` calls.
 This library ensures that logged messages do not overflow Android log message limits
 by efficiently splitting messages into chunks.
 
+## Consistent log filtering in mixed Rust/C/C++ apps
+
+Android's C logging API determines the effective log level based on [a
+combination](https://cs.android.com/android/platform/superproject/main/+/main:system/logging/liblog/properties.cpp;l=243;drc=b74a506c1b69f5b295a8cdfd7e2da3b16db15934),
+of a process-wide global variable, [system-wide
+properties](https://cs.android.com/android/platform/superproject/main/+/main:system/logging/logd/README.property;l=45;drc=99c545d3098018a544cb292e1501daca694bee0f),
+and call-specific default. `log` + `android_logger` crates add another layer of
+log filtering on top of that, independent from the C API.
+
+```
+    .-----.
+    | app |
+    '-----'     Rust
+C/C++ | '--------------.
+      |                v
+      |          .-----------.   filter by log::STATIC_MAX_LEVEL +
+      |          | log crate | - log::MAX_LOG_LEVEL_FILTER,
+      |          '-----------'   overrideable via log::set_max_level
+      |                |
+      |                v
+      |     .----------------------.
+      |     | android_logger crate | - filter by Config::max_level
+      |     '----------------------'
+      |                |
+      |   .------------'
+      v   v
+   .--------.
+   | liblog | - filter by global state or system-wide properties
+   '--------'
+```
+
+`liblog` APIs introduced in Android API 30 let `android_logger` delegate log
+filtering decision to `liblog`, making the log level consistent across C, C++
+and Rust calls.
+
+If you build `android_logger` with `android-api-30` feature enabled, the logger
+will consider the process-wide global state (set via
+[`__android_log_set_minimum_priority`](https://cs.android.com/android/platform/superproject/main/+/main:prebuilts/runtime/mainline/runtime/sdk/common_os/include/system/logging/liblog/include/android/log.h;l=364;drc=4cf460634134d51dba174f8af60dffb10f703f51)
+and Android system properties when deciding if a message should be logged or
+not. In this case, the effective log level is the _least verbose_ of the levels
+set between those and Rust log facilities.
+
 ## License
 
 Licensed under either of